This is a good sign - they think your software and their expertise, whatever it is, can make money. I'd recommend that rather than selling it all for $6MM, that you evaluate it for a couple of days, and if you are comfortable with them, suggest selling 60%, perhaps for the same $6MM, which gives them the control they seem to want, and retain 10% each, with the option for each of you to negotiate a sale of that 10% later, with them getting first dibs.

If the product does very well, it'd be good to have some skin in the game. If it fails to grow as much, then you have secured some upfront cash.