> attractive salary

I guess you really mean "hourly rate", because salary means being employed.

> i have my own two-man company > They offered shares and assured immediate budget for another developer

This sounds like an aquihire. So calculate what your own company is worth, add how much you earn within one year by staying your own boss, and round up to a lumped sum. Ask for this money in cash, and not in stocks, ask for same in stocks in addition, and add a good salary.

Their alternative is to continue paying you a hourly rate. But its your decission if you give up your freedom to become a wage slave again.