Multiple Tempo Texts simultaneously

Tempo is connected to time. What you are suggesting would be like playing two different CDs simultaneously. Voices and lyrics are usually different based on rhythm within the same time signature. I don't know if MuseScore could handle different time signatures for the parts. You might indicate when the second performer should start and end in relation to the first performer and their relative tempi, but I doubt you could get MuseScore to play it the way you want it. You could have the parts in different files and save them as .PNG files, then use a graphics program to combine the staffs. You could export the original two files as MP3 and then play them simultaneously.