Just thinking aloud.  Assuming the two installations already point to different paths for client data, perhaps two separate login for each user?  Presumably, you'd have one domain name for each of the firms.

I has been more than a decade since I did this. I think you simply install both prep files. In Screen 1 you select the Firm Number right about Preparer.  You would do well to contact support if you have any problems with this method. 

I did find this https://proconnect.intuit.com/community/help-articles/help/installing-multiple-firms-or-preparer-fil...