Splitting batch into IPA and Black IPA

want to make 10gal all grain batch of an IPA and make half black.

whats the best way to do it? steep a half pound of Carafa 3 in half gallon of water and boil/chill it separately, then add in the fermentor? should I adjust the amount of carafa up since it will have worse extraction?

I´ve never done it but if you are already splitting the batch why not cold step (like cold brew coffe) some dark grain, you don´t need to use carrafa, chocolate malt or black patent will work fine, cold stepping those grains will give you a less astringet roasty flavours closer to carrafa than chocolate malt. If you do this ahead of time (cold stepping requieres more time) you´ll be able to taste it and tweak it as neccesary.

When I did a black IPA I used carafa but added it at the sparge (think hot steep for a short period). Was happy with the results the goal for black IPA is to get the color and a bit of the flavor but not the roasty and astringent flavors. I think I'd try the cold steep and boil separately for 15 minutes. On the other hand I probably wouldn't do this. I'd want my black IPA to be significantly different than my regular IPA and don't think I could accomplish it this way.

Black IPAs are a "new" style an as a new style I think is still defining itself. There is black IPAs that are only coloured IPAs with very little roastiness and dark malt flavours, there is others (like Emelisse black IPA) that are actually closer to an Imperial Stout than an IPA. I like better to use a little uk chocolate malt and carafa in my black IPas and not only Carafa, I have only brew two batches of Black IPAs so I´m not by any means an expert but I like a little more dark malt caracther in my brews without going over the top. I defenitly going to cold step some chocolate and black patent next time and put a little carafa in the mash. Don´t actually know when I´m brewing this (I have a lot of things on cue) but this is the way I´m going.