Depends how well they get on. Our old yard had a 22X10 field shelter (actually field stable - it had a front on it) split into 10 and 12 foot sections. We have 2 horses, one Sec D at 15.2 and a longish TB at 16.2.
One day when it was really nasty we found them side by side in the 10X10 section both munching on the big haynet in there. Apparently they were in there for a good 4 hours whilst the rain was lashing down. So we could get away with a 10X10 if push came to shove but they really do get on like brothers when they have to...
thanks - they have only been togther for a few weeks and get on okish at the mo - the big one is a chunky cob and the little one is fine built so he really could do some damage if he wanted to - he prob would if food was involved coz boy does he like his food - or anyone elses given the chance! Ive been looking at 16 x 10 but wondered if thats too small?