Here is something I have done. I have a test user account for when I want to investigate a send/receive or shared project problem report.
Find your user settings folder, which is usually in c:\windows\users\USERNAME\appdata\local\Paratext93.
(USERNAME means your WIndows login user name. The last folder may be Paratext91 or Paratext92, depending on your installed version. Paratext93 means Paratext 9.3)
The registration info is stored in the file RegistrationInfo.xml.
So when you are logged into Paratext as User 1, copy the RegistrationInfo.xml file and name the copy User1Info.xml. Now when you use the registration info to change to the second user, when you finish close Paratext, and copy RegistrationInfo.xml to User2Info.xml.
Now when you want to switch IDs, copy the User Info file from the file you want to switch to, naming it RegistrationInfo.xml (replacing the existing file). When you start Paratext it should be using that user info. It will still go through the process of “Preparing Paratext for a new user” (I am not sure all that is involved).
But remember what anon848905 said about doing send/receive. One thing that could happen when you switch users is Paratext might say “you are an observer in Project X, all changes have been discarded”. So don’t make changes and forget to send them before you switch users.