Maybe I misunderstood your question. There are accounts and there are users within accounts. Two people sharing data will need to use the same account, but should use separate users within that account. That's how it's designed to work and that's what the pricing is based on.
So when you asked "can both be logged in on one account," the answer was yes. If you had asked "can both be logged with the same username," I would have said no. Does that help?
The only difference between a Solo account and a 2-5 user account is that you can add more users to a 2-5 user account. You won't have to learn anything new, unless you get into the functionality for customizing or limiting what different users can view or edit.
The need for migrating data depends on what you have now. If one of you already has the data set up in your account, that person should add the other person to that account where the data is. If you both already have the data set up in both accounts, you should choose the one with the best data and use that and abandon the other. If neither of you have any data set up in either account, you should choose which account is most suitable for this project (typically the account owned by the person who has the greater long-term commitment to the project) and add the other user and whatever data you need to that project.
Now let's say you decide to use person A's account for this project and you add person B to it. Does person B still need an account? If person B is only interested in accessing the project owned by person A, then no. If person B has some other projects that don't involve person A, then yes. In the latter case, person B would log into person A's account to access person A's project and log into his own account to access other projects that don't involve person A.
I'm afraid this explanation makes it sound more complicated than it is. I'm trying to spell out all the options because I don't know your exact situation.
Topic: Sharing wih another solo account holder. (Read 19639 times)