Generally speaking AWA will take more than one J.S. if they have available seats. I Have heard this applies mostly to airlines that also take more than one jumpseater although I have never not gotten on one of their flights. Just be careful as this time of year the flights will be pretty full- especially on weekends
I could be wrong, but the only airlines that I know of that allow more than one jumpseater (or two if the equipment has two) are ATA and Southwest. I have ridden on America West and I am pretty sure that they only allow one per aircraft. Also, if you are not an AWA pilot then you can not ride in the jumpseat so if the cabin is full you lose.
One last note, I have jumpseated to mexico and you have to pay a tax which was around $40 bucks. Good luck.