Can a shared user use 1 app for two accounts?

I want to share my account with him but he already has one with another company. Can my submissions still be seen by him if he is already logged into his other account on his Jot Form app?

I am not sure if I understand your query correctly. If you have already shared the forms to the other user, he should be able to view them in the 'Shared with you' folders when logged into his account. Please refer to the guide below that can help you with more information on sharing forms.

https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User

Or, are you referring to the JotForm mobile app? Please provide us with more information so that we can assist you further.

Thanks!