It is possible, but it is also very possible that you might have to split up. There are only 2 wheelchair/
ecv spots on a bus, so if anybody is in line in front of your party or already on the bus, only one of your parents will be able to board.
Plus of the Poly is the monorail, definately! Downside is that they sometimes share busses with the GF and Cont. Don't know which resort the bus will go to first, but if the Poly isn't the first, this can increase the chance of not being able to get both on the bus as there will already be somebody onboard with a wheelchair or ecv. AKL only shares with AKV, which is a plus. Downside; it's all busses.
Don't stress out too much. Stressing out tends to make one a bit more sensible to things possibly not going perfect. With 2 ecv's and 10 days, there probably will be a moment where both wont be able to get on the same bus. You can either rent a van or decide it wont break a trip and the family having a great time and split up where needed and meet back up later. You could also decide to skip the bus when you find out there's only one place left, but when the busses share and you're not at the first busstop you do run the risk of the next bus also having only one place left.
I tend to pick resorts that don't share busses and/or aren't too populair with those using a wheelchair or ecv. CBR is one of them, most seem to loath it. This is why I love it!

Beautiful landscaping, great atmosphere and not as much wheelchairs/ecv's waiting for busses as I've seen at busstops for other resorts.
Another option is to look at how badly they need the ECV and what your parkstrategy will be. For instance; if one of them would need the ECV mainly for the long distances and you hit the park pretty early, it might be an idea (if money allows) to rent one ECV at the park. It is easier to get on the bus with only one ECV. The 'ecv-less' parent can board with the one with an ecv, therefor insuring both will not have to stand on the bus. This is offcourse only possible if it fits all of your families needs.
For next year, I'm renting DCV-points. I've decided to go with AKV, because of the share issue. I'm a powerchair user myself. That trip, I will be travelling with a friend that will use an ECV. We know there is a chance of not getting to ride the same bus all of the time. No biggie, we can always meet up somewhere later on. It's nice to travel together, but it's not a necessity for us to enjoy ourselves.