Sharing lots of signals between micro-controllers
Two-layer PCBs are really cheap.  If cost is the primary facto I would use 2 X 2-layer boards connected by headers.

A preliminary quote from PCBWAY shows you can get you 10 pieces of 3.5&quot; x 3.5&quot; boards for $5.
Keeping all else equal and changing to 4 layers makes the price $49.

https://www.pcbway.com/orderonline.aspx

You could either put one MCU on each board.  In this case you may be able to make both boards identical in layout.  This could save cost.

Or you could just use the second board for routing.