Partial boil on a 10 gal IPA

Hi folks,
I have been doing all grain 5 gallon batches for many years, but recently came across an extra kegging system that I am giving to a friend as he starts up. He doesnt have any other equipment and I'm not quite ready to buy my 15 gallon SS system yet, but I want to get him hooked so I'm thinking we can brew some double batches and split them into two fermenters and then he can take a keg of homebrew with him a few times and then he'll never look back.

So, to execute my plan, we need to brew 11 gallons of beer in a 7.5 gallon boil kettle. thoughts? Can I get enough sugar out of a concentrated mash to split 3 gallons into 2 fermenters and add 2.5 gallons of water to each? Or am I better off doing a partial mash and adding extract? If the latter, how should I split up the grain bill/extract? Specialty grains in the mash and 2-row extract?

Are there any recipes for this sort of thing? I'd like to brew a big hoppy IPA as that is what he likes, but I definitely don't think I could get enough grain in my tun for a high enough OG to then dilute it by half. I have a big orange igloo tun.

Thanks!

Why not just boil the extra wort in another pot at the same time?

You can do a partial boil, but there are a couple of caveats.

One is when you're making a really hoppy beer, unfortunately. The issue with a partial boil is sort of complex, but the gist of it is that only a certain amount of hops oils can isomerize in water. The papers I've read and heard about say that this limit is approximately 100 IBUs. That's why even the hoppiest beers are about 100 IBUs, even if they calculate out to 200+ (Pliny is a good example).

That's usually fine, since the human tongue is unable to perceive any more than that anyway. But the issue then is when a partial boil is done. Say you end up with 5 gallons of 100 IBU wort for the IPA. Just the act of adding 50% water will dilute those IBUs to 50. There just isn't any way around that. You can get the correct OG by making a more concentrated wort, but you can't get the IBUs higher.

You can make great beer with a partial boil, but I think IPAs would not be one of them due to the limits of hops oils isomerization and this IBU issue.

The one thing that might work, although I've never tried it, is to buy some hops oil extract (I think it's called something like Hopshot). That would not be subject to the same limits, as it would be added to the wort. It might be worth some research if you really want to to 10 gallons of IPA in a small kettle.