I'm going to migrate two SUP32 cards to two RSP720 cards. Curently, we have two C7604 (iBGP session between them, one active and the other one for backup with one SUP32 inside. To make some tests on the futures RSP720 cards (iOS "installation", pre-configuring, link tests...), I envisage to plug the new card in the backup C7604.
In Cisco Documentation, I find that you must insert RSP720card in slot 1 or 2:
Both supervisor engines in a single chassis must be identical.
Slot 1 is reserved for the supervisor engine, which provides switching, local and remote management, and multiple GBIC uplink ports.
Slot 2 can contain a redundant supervisor engine that can act as a backup if the first supervisor engine fails. If a redundant supervisor engine is not required, slot 2 is available for an OSM or other supported Catalyst 6500 series modules.