I've not known 2 people on a phone contract, but I can't see why not providing you are both prepared to be jointly & severally liable for it/them.
If it is possible, you would both have to undergo & pass a credit check, which begs the question:
Why do you and your gf want to share a contract? Why doesn't she get a contract of her own?
If you want to pay the bill for her, then she could send you the bills

(or vice versa)
If she has poor credit history, or is under 18, and so cannot get a contract of her own, you would be denied on a joint account (if that is possible)
You could, of course, take out another contract phone in your name, give it to her and send her the bills for it - only problem is, if she fails to pay, vodafone will come after you (but that would happen if you were jointly & severally liable anyway)