heh...been there, done that, bought the t-shirt.
A very similar thing happened to me. I spent a very long time trying to get a hold of my first group before giving up and contacting the second group. Just when sorority #2 responded to me, sorority #1, now *8 months* later, decided to up and e-mail me out of the blue!
Since sorority #2 was still trying to work out which chapter I should contact, I went ahead to the event that sorority #1 invited me to attend. Though, for personal reasons, I ended up withdrawing my interest in sorority #1 shortly after. About 2 months later, sorority #2 got into gear and invited me to their events.
I didn't mean to contact two groups at once either. However, unlike you, I wasn't actually invited to events running simultaneously. Hrm...it *could* get sticky if people find out that you're looking at two groups at once. It depends on the groups and/or individuals you're in contact with. I noted that you appear to be located in Texas - if so, this is even more important.
I would recommend the total honesty approach. Explain what happened to both of the alum presidents and hope that they're understanding. Otherwise, you have two other options: decide now which one of the two groups you want more; or, attend events for both and hope they don't find out and get upset.
I hope everything works out for you!
PNAM,
.....Kelly