2013 trashcan Mac Pro internal gpu issues with egpu
good question...what i can say is, the A+B cards are different. while card A is holding the connection to the HDMI and TB ports to display something, card B is holding the SSD as well, so if you loose the "wrong" card, means you cant boot the mac pro with the internal drive. while driving macOS, one card is always in idle and just kick in, when there is a need to do so.
i was looking before for an "upgrade" to a dual d700 but when i see the prices in the bay...until i found a used 6,1 for a reasonable price with d700 and really cheaper then the cards alone, but i decide to go the egpu way.