Is it possible to browse the site and have it only display images that have both 'tag X' AND 'tag Y'? It seems like it should be possible, but I can't figure out how to do it.

Sure. Searching for say.. "gay" and "female" will only show you posts that have those two tags.

So you just separate the tags to search for with a space? (as opposed to a comma or something)

Yep.

'K, that works for two. But any time I try AND'ing 3 tags, it just goes back to http://e621.net/post

By default the software has a 2-tag limit for Member-level users to reduce CPU load.

See also forum #63

a cheat:

some tags have two words separated for a "_":

cum

cun_inside

you can write "oral cum_inside" and it search

PD:Sorry my english