multiple batch question

I am planning a fairly large brew day tomorrow (3-4 batches). I had the idea to mash a rather large amount of base malt (20-25 lb) to use as the base for 2 beers and then steep the specialty grains similar to what I would do if brewing an extract brew. Would this work or do I need to have a separate mash for each of my different beers? Thanks in advance.

I think it will work, I plan to do this same thing myself in the future.

It will work. You could also do a mini BIAB mash with the other grains. I did this with a combined brew day that included a kolsch and an altbier. My kolsch was 100% Pilsner malt and my altbier was Pilsner with some Munich and roasted barley. I mashed enough of the kolsch base to make 2 brews and did a BIAB for the Munich and roasted barley in a pot on my stove (this was happening while I was boiling the kolsch).

I'm drinking both of these currently and they are tasty.

Sounds like a good idea, actually.