I'm not Gregg, but since he hasn't replied yet, I will...
SAIMR (South African Institute for Medical Research) makes a polyvalent AV that covers Bitis, Naja, and Dendroaspis pretty effectively. This would be the A/V used to treat both rhino and gabby bites by either one, so it's pretty reasonable to guess that it would be used for a rhino/gab hybrid as well. Hope this helps (and sorry for stealing your reply Gregg, but i couldn't help it

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AP