We're going after work tonight - I will definitely ask why separate booths if that is the case in my precinct. You should be able to use whichever booth you want - as long as your ballot goes into the proper box, which should be separated according to party. If it's a machine lever voting booth, the ballots aren't interchangeable. If it's a punch-card system, the voting guide books aren't interchangeable.
Well, here in my city on the Central Coast, we're still using paper ballots that you mark with a black magic marker. Maybe someday we'll get into the 20th century (the 21st may be too much to hope for.)
I was amazed when I moved out here from Taxachusetts - I lived in a little one-horse town and had been using voting machines from the time I registered back in the late '60's.