I purchased a pair of full sisters with the intention of putting them together as a pair. They are the same height, but they are built somewhat different.
It took time to get them working together - started in February 2010 and by October 2010 (both working on my own and working 1x week w/ a trainer/instructor) had them working fairly well together. At that time, I had a wagon, but not the appropriate harness to hitch them to it. They pulled great together in the home crafted, pleasure style harness I made. When I got the new harness in March of 2011, it took more time to get them going. I found the longer, leaner mare, Bell, had a slightly different mode of travel - longer and low - and would drag her sister. I learned to use a "buck back strap" (draft term - a separate line that runs to the bridle and then hooks to either a trace chain, the evener or to the wagon itself) on her while also using voice and whip to encourage Bit to "step up". We slowly came around...
Taken 29 June 2010 - 1st time ground driving as a pair. They've each been ground driving single & hitched since May. Bell foaled on 10 June.
Taken September 2010 - each is driving single very well. Bit gets upset and will rock back on haunches - semi rearing and/or doing Lipizzaner moves (has been exciting a few times!!).
Then in 2012, I got a new wagon - smaller, lighter and I thought easier for them to pull. Certainly nicer for me to sit it comfortably while driving for anything longer than 30 minutes. But had problems w/ hook up right from the start. I took it to TX to give my Dad and the others in the home he is in rides for the day as well as attending a show there. I was able to get the Draft Style aficionados to give me a hand with the hook up! They also convinced me to enter my pair in the show - the only entry in their class (they were ASPC not miniatures - there were a couple of teams there for the Mini draft classes).
Well, in the process of doing this - I also have trained several others to work with the original pair(Koalah, Cassie, GG. Then others that are smaller are working together - Iggy, Stuffy, Cupid, KoKo - in different configurations). I have found that they all get to working together - if you drive them enough together. Also, while working with them - I lead them everywhere as a pair. I catch them in the pasture together and lead them in - the ones in a different pasture (gelding in geldings, stud in different pasture) get placed side by side while grooming. They stand next to each other to be groomed and harnessed. They ground drive a lot together. I even did a lot of "down & dirty" driving - where I didn't have time to fully groom/hitch a pair. I would simply tie them together using a long leadline - from the neck around the butt of #1, around the butt of #2 to the neck of #2, a strap between their halters or bits & 1 single line from outside of bit of #1 to outside of bit #2 - would ground drive them in/out of our barn while others eating, drive around in my barn when raining etc. Even if only 10 minutes - it MADE A DIFFERENCE, as I learned to work with them and they learned to maneuver. When ready - they get hitched. I drive them around our place a while - then hit the road and after checking for traffic (on a low traffic road) - we head out on a 2.3 mile loop at a trot. They trot until "we" are tired. That gets them working together really well - they learn to adjust better/faster at a trot (for me).
One mare is 3" taller than the other two (Koalah) - but has a similar work ethic and moves in between the two sisters. She will now work with either one or w/ both in a 3 abreast hitch.
Koalah (larger, round mare on the right) - Nov 11. First pair hitch to wagon. Started ground driving in February and hitched single to cart in May - with her April 2011 filly trotting alongside.
We had to go back to using the "buck strap" - I carry it with me whenever I take the wagon or forecart (3 abreast hitch) out since I never know when it will come in handy and keep me from wearing myself out trying to keep one held back while sending the other(s) forward constantly. It varies as to who will need it - the weather, how they are feeling, which one is in better shape, a "wild hair"... And now I'm ground driving a 4 abreast hitch!! I can't however hitch them by myself - so will have to coordinate help to do that in the future.
Now, I often drive Bell & GG together. They are mother and daughter - but GG is almost 2" taller than Bell. They work well together - GG used to go to the original driving/training lessons (see 1st pic). They are a joy to drive. They seem to look forward to driving - and always have their ears up and while have plenty of go, they respond easily as well. Pic below is GG's 3rd pair hitch with her dam, Bell, to the wagon. She'd been only driven to a cart single 5 times (at that time). She will be 3 in June.
I was planning on putting GG & Cassie together once they both got "more solid" - but just sold Cassie and her daughter (not driving yet). Bought a new mare that is larger still and will match her daughter that I purchased almost two years ago... So as "permanent pairs" - I will drive Bell & GG together and Koalah & Bit together. Bit has finally lost the battle with constant eye infections and we are discussing leaving the eye in it's socket or removing it. She can't see out of it and it has shrunk in size. She will continue to always be driven on the right - where she is more comfortable (I used to switch them out)...
What matters more than size is their work ethic, wether they "get along", and then logging in the mileage for you to become familiar with the handling of a pair and them to learn to work together. Even different striding ponies will learn to match up - eventually - UNLESS one is just bound and determined not to work as a pair or gets upset all the time.
Sorry about my book! I seem to do that. I'm going to try to attach some pics - the last couple of days I haven't been able to. IT WORKED!!
Because I am constantly training "new ones" and have a WHOLE CREW of youngsters bred from the original mares - I spend a LOT of time ground driving. Right now, I have several that have been hitched and will work together, but they DO NOT have the driving time that Bit, Koalah and Bell have. It makes a HUGE difference in them learning to work together... and becoming a "working unit". I've started "weeding out" my ponies - looking for the ones that are happy trotting down the road and pleasant to work with.
I really enjoy driving pairs!!