Checking for discs while another player is playing

It appears that checking for discs while another playing is not a good idea.
If I have CDJ check the player that is NOT playing it seems to go into a
loop of many many passes with no apparent progress on doing the inventory.
It keeps the changer moving so it is trying.

I have CDP-CX240 and CDP-CE515 on the same S-link bus. The 240 is
CD #1 and the 515 is CD #3.

Incidently, is there a limit on the number of passes CDJ will try?
I haven't had the guts to let run for too long. Today it appeared
that CDJ got stuck trying to check an empty slot on the 515.