I tested collocation by filespace but it did not use both tapes, well the test 
is not very fiable, I am not too sure what it. It is very complicated to 
interpret the activity log with all this sessions, but only one drive had a 
tape mounted.
I have decided to define two clients in the same machine and distribute the 
filespaces using domain statemets among them.
This appears to do the job we want. We will see on Monday what has happened.
I imagine that now we have to modify resourceutilization for only 1 and 
maxmountpoint 1and of course collocate yes
Thank very much to every one who helped.
Maria