it seems incredibly unfair and unreasonable that those are the terms of their offer. i believe what you're saying, but that's just not right, in my opinion.
I know that you're not exactly in a position to press this other firm for information, but if I were you (since you've got nothing to lose right?) I would talk to the recruiter or whoever your point of contact is, and really ask them about a long term gig there. You should state that it's a huge risk for you to take on to give up your current job and accept this new one, when there's no guarantee of it lasting more than 2 months.
If they won't give you anything more than "we'll see after 2 months" then as much as it sounds like a dream job, they may not be the best people to work for. Any recruiter should understand how ridiculous that is to expect somebody to drop their job for only a 2 month commitment.
I hate to be the adult in the thread, but it's too big of a risk to take if it's only a 2 month deal. Just my opinion. I think you should call, not e-mail, but call the person you're speaking with and see what the long term prospects are. Make sure you explain why you're asking and concerned, as it's a very reasonable thing to ask.
"I'd really like to get my name out so I can piss on 13 year old girls and get away with it, do a shit load of heroin, autograph boobs without it being harassment and then finally blow my brains out when the fame gets to me."
"i was going to post something demeaning and funny about the op, but then i saw that TechnoPotamus did so now i find it unnecessary."
"Professor Potamus has a PhD in Newschoolers."