I think what I'm going to do is just round-robin my gravity skill ups. When I think about it, I usually use them in groups, so it makes no sense to have one ready but still have to wait for the others.
Plus, I really don't want to deal with having a max skill and then feeling like I'm starting over on the next monster.
That's kind of what I did (and it's very painful). Since you basically have to stall for your longest skill in most cases, I have Is and D/D Hades both at sk.3. I'm thinking i'll do 2-3 skillups before switching.
So basically a billion Hera runs.
I'm on my 10th run today with only one stone used so far. Was having the same thought about who to feed first or if I should feed both. DQ Hera has better stats with less RCV than IS but IS fits in better on my Horus team. It's a tough call, I'm probably going to try and level both like you guys are saying.