Just use one vial for each cat (each vial is meant for one cat per month).
You'll have to pick up some more for next month for one of your cars though, but having a new source of income should help with that.
Congratulations. :-)
You probably could, however there is such a tiny amount in the cat vials that it will be very difficult to get it divided up evenly, so chances are somebody is going to get really shorted. As it is, even half a dose is going to be very questionable unless your cats are very small. I suppose it's better than having no protection, though.
How long ago was it that you dosed your cats? If it was only a month ago, you still have time left because FrontLine TopSpot cat flea treatment is good for six weeks, not a month, so if you use the half dose on each of them, they should both easily be protected for another 30 days until you can afford to purchase another 3-pack. OR, if you think you will be able to afford another 3-pack in the next two to four weeks, just hang on to that single dose you have and you'll have to FULL doses when you buy the three-pack. As long as your cats don't get bathed frequently the extra two weeks (past the six week guaranteed efficacy period) shouldn't be a problem, they should still have some protection.
Ghilly