Based on this thread:
http://forum.team-mediaportal.com/installation-configuration-support-53/mulitple-dvb-s-tuners-85199/

When you tune to a channel some brands of cards have to do a translation of the tuningdetail info.For instance a ProfRed card has to set the Modulation to ModNotDefined instead of ModNotSet.

This however breaks same transponder checking.

Take the following scenario:
Tune to channel 1 on transponder 1.
Modulation is translated to ModNotDefined.
Try to tune to channel 2 on transponder 1 from a different client (or start a recording).
This will check if it is the same transponder as currently tuned to. This detail has ModNotSet, which does not match the translated ModNotDefined. So card allocation thinks that this card is not usable because it is tuned to a different transponder (modulation in this case).

h4. Additional Information
The translation is done in:
ConditionalAccess.cs.