These combinations could only happen between October 1, 1935 and December 31, 1935. After that, all of the San Martin stamps were retired from postal use.
Here is a cover from December 1935.
Here is a first day of postal use (October 1, 1935) cover.
This cover was sent to Barcelona, Spain.
Notice that the 10c Rivadavia Red is a Type II, and this means that both types were available for postal use at the same time.
On the back, the cover has several interesting markings: a Sevilla slogan, a Por Avion Barcelona receipt mark, and a 'LLEGO POR VIA TERRESTRE' (arrived by ground) marking that makes no sense since this cover should have travelled by air all the way to it's destination.
This last cover is to Paris, France.
Here are the back markings, one for departure from Buenos Aires, and another for final arrival in November 26? (I think it's a 6) in Paris.