I think it's unreasonable for an agent to ask for an exclusive on a partial--especially given the reality of submitting, which is that it's more than likely that any writer will have a number of partials out at any given time. Speaking for myself, I wouldn't grant exclusivity on a partial. I'd simply send the requested chapters along with a note that someone else was reading (even if they weren't). If an agent is interested enough to ask for chapters, odds are he'll read them whether or not he gets them on an exclusive basis.
It sounds as if you did grant the exclusive, though, so you'll have to put Agent 2 off. Just send a brief note to let her know that someone else is reading and you'll send the chapters along when they're done (when you do send them, be sure to include a cover letter mentioning that she asked to see them--if several weeks elapse, she may forget).
You should always ask for a time limit on an exclusive. If the agent won't provide one, give him one yourself--IMO, two months is reasonable for a full manuscript, three if you want to be really really kind and tolerant (some agents ask for even longer than this). For chapters, I don't know--I'd think two to three weeks would be enough, four at the most. If you haven't heard anything by then, write and ask (gently) what the status is. If you have trouble getting a response, or get put off more than once, give the agent an ultimatum: if you don't hear anything by such-and-such a date you'll consider yourself free to submit elsewhere. Agents are really, really busy, and even--gasp--have personal lives, but you can't let these guys string you along forever.
I should say again (actually I don't know if I ever said it) that I'm not an agent, just a writer and writers' advocate who observes agents and their habits very closely. Sort of an agentologist. A real agent might disagree with any of the above.
- Victoria