I can describe the algorithm that should do the trick for you:
1) Get a library, using Get-VBRTapeLibrary
commandlet. Assign result to a variable.
2) Get all drives present inside the library, using Get-VBRTapeDrive
commandlet and the variable gotten at the first step.
3) Disable all drives, but needed one, using Disable-VBRTapeDrive
commandlet and drives names as distinguishing factor.
Don't forget to re-enable drives
either via post-job activity of the first job or via pre-job activity of the second or third job.