I have them both as well and often both of them have a 0-20% battery in the evening. I use one for reading while writing in Nebo on the other big one. Or the 9.7 is my to-do-notepad and I'm typing away at the 12.9.
The copy across your devices is brilliant for that!
When I'm studying or working at home, office, on the road I have both with them.
When I'm out and about, I have the 9.7+P+ASK in a smaller bag. Pint of water/lemonade, sandwiches, small monocular, Shure Se846's and a few other nicknacks in a canvas/leather satchel and I'm good to go for hours or the whole day.
Come to think of it, between family, being self employed for 15 years now and a full four year university study in the evenings I hardly ever get out these days. Let alone a whole day.
But it's nice to be able to do so
What is your use for having both?