BackupExec 2010 asking for 2nd tape reporting 1st tape full when Job is only half of capacity of tapes

you must have set the job to append and the tape might already have some data written to it . You nees to set the backup job to overwrite when it begins and make sure you have overwritable media>

.....and if you are calculating the "half of the capacity of tapes" w.r.t media capacity in compressed mode, then it is wrong. Very rarely (can say never) we can see jobs achieving 2:1 compression ration. 1:1.4 can be considered as good compression ratio.

If Backup Exec is reporting that the tape is full, then it provbably is full.  Backup Exec writes to a tape until the tape drive tells Backup Exec to stop due to the tape being full.

Specifics about your tape drive & cartridges would be helpful.