Wheelchairs are not that comfortable for putting kids in (or anyone who is not at least 5 feet tall). The kids I have seen in the park rental wheelchairs look very uncomfortable. The seats are plastic sling seats, with plastic sling backs. So, it's similar to putting 2 kids together in a director's chair - they slide to the middle and end up kind of squished together.
Plus, the armrests will turn out anywhere from mid-chest to neck high; not comfortable to rest arms on. And, because the seats are deep enough for an adult to sit, if the kids sit all the way back, the bend of their knee comes before the edge of the seat, so their legs are stuck straight out. If the kids are sitting far enough forward that their legs can bend, they have no back support.
If you need to economize, it would be much more comfortable for the kids to trade of using a single stroller.