I work FT, my Mom and my MIL split the childcare for me. My 4 y/o has been going to one or the other ever since she was 3 months old, and now she is in her 2nd year of pre-school (a few mornings a week). So she has 3 different places she is going. My one year old is just with whichever grandma for the whole day.
It's been fine :) Some days I don't know if I'm coming or going because they'll start at my house, then take them back to theirs, other times I wake them up early and drive them straight to grandma's, it depends on the day. This year Mom and MIL decided they each wanted to be responsible for 2 days, then they would literally split the middle day in half. So on Wednesday my girsl start off at one grandma's and end the day at the other's. For the older one that means school in between too!
I know what you mean about what they do all day though... My MIL seems a bit more hands on with the kids and is likely to have made a play-doh project, baked something with them, taken them in the pool or played boardgames. My mom is more likely to have them just running around the house and entertaining themselves with the toys that are dumped out everywhere. Neither of them seem to like to take them out too often! They have to though, or they get cabin fever. They will take them to the park, or my mom likes to drag them around to visit her other lady friends. Your mom would have to take her out of the house eventually :)
I was glad when my daughter started pre-school to be able to socialize with non-family members. Even though it's only a few mornings a week, it's been great for her. I think pre-school time plus grandma-time is the perfect combo while I'm at work.
Anyway it all works fine, we adapt and develop routines around the childcare. My evenings are like clockwork no matter where they went that day.