MerlinsMage
I would also say that you could toy with the idea of them joining by phone if it would work for you, but if it's Undergrad or Work keeping them from it, probably not. My old DM did it though. He would call, put us on speakerphone and let us play that way...we just had to pick someone to roll for us. We still got to join in, still got a chance at playing and the only thing different is that it's harder to assert yourself to an a*****e player. lol. It worked out great for me since I was sick for an entire summer of play.
We've used Skype before and had good experience with that, but one of the players in question is at work, and the other is in class, I think.
Lazarous
If they want to work for it, tell them that they can have the exp if they do something for it outside of the sessions.
Say you leave one of their characters at a small village while the party members present go off to the regularly scheduled dungeon. The missing player might draw a picture or a comic strip of their character cutting wood for an elderly villager. Or they write a short story about how they catch the hooligan that's been beating the village dogs. Or they write a little character background, or they write some of their character's thoughts on what's been going on.
Stuff like that. It doesn't force them to do something, but it adds value for the GM and coincidentally perhaps some introspection about their own character.
We give extra credit XP in the form of posting things like that (with a given "homework assignment" from me so that if I want specific information to guide the next session or something, then I can get it easily.) on a forum site that the previous DM set up because he knows how to do things like that. (I don't.) It doesn't usually add up to enough to make up for what the missing characters would miss, though, so I'd have to give them their own special things. Which I could do, but one had mentioned specifically "I don't want to have to write a novel every time I miss so I can catch up."
The previous DM used to not give uniform XP, and I don't think 3/4 of the players would mind not getting uniform XP, but one of them would probably complain incessantly about it if I didn't give him at least as much as everybody else (even though, for example, last session he fell asleep for an hour or so mid-session because he felt that one of the other characters was making his character un-usable). So I'd have to keep it secret, and that makes trouble for people wanting to craft things. :/
I would probably also get accused of favoritism, even moreso if I keep it secret, because one of the players is my boyfriend. Regardless of the fact that he's one of the better players (though last session I would put him at 3rd place, because two of the players got in a fight in-character over whether or not to kill the Gargoyle, since one has taken Vow of Nonviolence & Vow of Peace and the other is a LG Paladin of Heironeous) I worry that any time I would tell him to level, at least one of the other players will question if he's leveling just because he's my boyfriend. :/