God Bless! Please don't worry about how much you get accomplished; enjoy your time with them. Take thousands of pictures everywhere and treasure your grand aunts. This was an inspiration to me.
I would go to guest relations and see if they can do anything special for them. If it's there first time there they will give them each a button at least.
I had my mom there for the first time when she was 87. I had her wheelchair decorated with her age and it stated it was her first time there. I didn't expect anything and then to our suprise, a CM member noticed her sign at the Buzz Lightyear ride and she asked us to wait a few minutes. Well, she returned with a special, handwritten yellow slip that gave us fastpasses to three other attractions. We each got a free ice cream, too. She got a button for her first time there. We were allowed to ride Buzz three times in a row, as well. My mom felt very special in that moment and I won't forget it. It was special to all of us!
Hope you get some pixie dust, too; although, you already have some with them being there.
On a more serious note: If you find they both need
ECVs by all means get another. If they have difficulty with the ECVs then go to a wheelchair for them for the day. Many use the wheelchair as a walker if they want to walk some through the day. Maybe, one whheelchair and one
ECV would do the trick. Take some rests like enjoying a meal, show, etc. Make sure you give them ample opportunities for restroom breaks. If you do any rides the ride queues can be very difficult for ECVs; there are hard turns involved. There are some rides where you can transfer to a wheelchair provided by Disney at the ride. Some rides allow the wheelchair right on the ride like It's A Small Small World; so, they will not have to climb in and out.