here. See also references to John Carter here and to ERBian Martians here. A search for Tarzan yielded results here. See in particular "At The Arizona Cave" here. Now I must track down the Barsoom anthology mentioned in the combox here.
At least some of Poul Anderson's diverse series coexist in a multiverse whereas ERB's coexist in a single universe:
the first Venus volume alludes to the Martian series and explicitly mentions the events of Tarzan At The Earth's Core which is both a Tarzan book and an Earth's Core book;
during those events, Jason Gridley communicates with Mars as he had already done with Pellucidar at the Earth's Core;
Beyond The Farthest Star and the Moon Trilogy are sequels to the Martian series;
all these works and The Land That Time Forgot are linked by common reference to the fictional ERB;
a character en route to visit Tarzan time travels in The Eternal Savage.
And we ought to see a green Martian in Anderson's inter-universal inn, the Old Phoenix. See here.