For those who says that Zim and Zam aren't human, you're onto something.
Nice brain teaser! I don't have what I would call the definitive answer. However, I have been thinking for a while that they might be Ostriches. Only because they are bipedal (you stated that they were standing on two legs), large (being that one was a foot taller than the other) and they cannot use the sense of feel or touch because they don't have hands and arms as primates do.
You also stated that they could have been there 10 minute or ten days. So, that rules out that it was in the dark of night on a moonless night.
Anyway, are we beginning to narrow it down?