i've known people that have had two. It works as long as your sharers realise they have set days and if they can't make them that's bad luck. It become tricky when they are no longer happy to hack and want to compete then there becomes issues over swapping days etc
When i started there was a girl who had been sharing him for a while, then after a bit she left and after a bit longer someone else came. They didn't communicate with me or his owner though, and didn't come up to sort him out when she couldn't ride (was slightly lame for a week or so) and didn't really spend any time loving him, just came, tacked up and rode, then chucked him back in the field and went on her way. So after a month or so she went too. Horse hated her too so he was glad to see her go
Now i've had him well over a year and a few weeks ago we found another sharer. She's lovely, experienced, we keep each other updated most days via fb or text, we have the same ideals and aspirations for him, and he loves her to bits. More than me. I need more apples!
It's great for him as his owner has several other horses and doesn't ride. When it was just me i struggled to keep him fit off 3 days a week so now it's great Hard to find the right person, most important thing is that sharers communicate and get on or else things will fall apart and the horse will suffer.