Hi
Good question.
There are definitely documents between co-founders. The most important one being a 'co-founder' agreement. I draft lots of them each month, and this is a very important agreement (especially considering that the #3 reason for startup failures is fights between co-founders).
You also want to be sure that the stock is allocated to you. This can be done by giving you a A Deed of Assignment of Stock Subscription, or by updating the company registrar of the transfer of stocks (shares) - but the process depends on which state the company is registered in.

Good luck

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