I thing you are asking if you can work in both companies at once. Yes, do it all the time.  Open Item Maintenance in one company, keep panel open, change companies and open Item Maintenance. I can now work in Item Maintenance in both companies at once.

Note you cannot have two instances of Sage 100 open at once. You cannot have your install of Sage 100 v2021 open and then open v2023

I believe what is going on here is that one company is converting data, as in when doing an upgrade or applying a product update. This is not something that I've generally seen done, but thinking about it..

My answer would be "yes". In theory you can work in one company while the system is converting another company. A couple of caveats would be that the company you want to work in will naturally need to already have been converted. Also, migrating and converting data usually takes a great deal of resources, so there may not be a lot of CPU and/or RAM available for your work.

SOP for most of the folks I've worked with would be to convert data when nobody else is in the system. Feels like a better safe than sorry sort of a thing.