Re: The below answer, I didn't notice I had dug up a really old thread, and thought I was looking at an unanswered question. Sorry to raise thread, but i think the below is still a valid suggestion for anyone starting out playing together for the first time. Anyway....
One thing that I ran into when I first started playing with other guitarists was that, depending on experience, the rhythm would usually fall apart, or be uneven, or other mistakes would make attempts to play together fall apart after a few moments. IE: a small hiccup in my playing rhythm would throw off the other inexperienced players and kill the song.
I found that using youtube to search for backing tracks IE: "Blues in A Major", or playing a CD of a backing rhythm while jamming helped in many ways:
1) You can still play rhythm over it, and by playing to a backing track, you will start to improve with keeping time, etc.
2) If the rhythm player makes a mistake or drops out, the backing track keeps the sound filled in, allowing the "lead" player to keep on while the rhythm player finds the groove and hops back in.
3) You can both play lead lines back and forth call and response style. Or share melodic ideas. Harmonize lead lines.
4) Eventually you can turn down the volume on the backing, and then completely get rid of it.
I think doing something like this will make your jam sessions more enjoyable at the beginning, and lead to better playing in the end.