stacks are nice, but why not limit the stacks to something reasonable, like only so many infantry or whatever in a hex. Give each unit a mass and dont let the hex have too much mass in it, either that or just set the limit to like 10 units max in a hex
A coop PC game pretty much has to have splitscreen unless users connect via LAN or internet. Its rare to find a PC game that allows for 2 players on 1 computer these days.