I believe this has been done 10 times (1-6*, 7.1,7.2,8.1,8.2), and the cameras have come back 80% of the time (7.1 and 7.2 lost at sea?). Two seems prudent.
*6 - Camera 6 I believe had two cameras shipped but they traveled together and for this purpose I consider it 1 camera unit (per International Camera Trading Society, bylaw 78.3a).
However, like Steve says, let's pick names for them! These 9.1 and 9.2 names sound like software releases, heck these are analog cameras! Just no polarizing names such as, colour, organisation, aluminium