Interesting idea. I don’t think you’ll hit 1.045 with the second beer without adding some more malt as you suggest. But also think about what you’re doing – you’re basically doing like a 2.5-hour mash for that second beer (except for the malt added halfway in). And this is a second smaller beer anyway, so it’s already going to be kind of wimpy. So that second beer is going to turn out even more wimpy if it mashes for a super long time – very thin and watery. I think the final gravity on it after fermentation would be like 1.002 or something since it is such a long mash time. What I might consider instead is whether you want to collect your third runnings right away instead of waiting an extra hour, bring up to >170 F on your stove in several pots or whatever it takes to stop conversion dead, and then you can worry less about it finishing too thin and dry. But you’ll need to add some serious steeping malts. My wild-butt guess is that you’ll only hit a gravity of around 1.035, maybe even less, and I’d consider adding more crystal malts or dark roasted grains and maybe even up to a half pound of lactose to help bring up the final gravity and give the final beer a little more body. If you don’t want a crystally or roasty beer, then a little extract might be the way to go to boost gravity as required. Best thing might be to test the gravity of the final runnings and then decide what you’ll want to do. Sort of off-the-cuff. But then if the gravity is only like 1.020, you might just decide to stop right there.
In a nutshell, I might sound skeptical but I really don’t know if this is going to work out, as there are a zillion variables, but it should make for an interesting experiment! Let us know how it all turns out if you do try it. I think if you were starting with a bigger beer around 1.100, then partigyle would be a no brainer. But it doesn’t look to me like your IPA is going to be quite that high of gravity.