Usually it's 2 male rattlesnakes doing this- to see who's the strongest to mate with the nearest female, but this is strange- it's a combat dance between a copperhead & a cottonmouth that was caught on camera a few years back. They're not normally competitors- so all I can think is that one of them is wearing the "wrong scent" & this is a case of mistaken identity? Anyway, have a watch & see what you think? They refused to speak on camera.... -that figures.
https://www.youtube.com/watch?v=YHA81Tfk3og