Bear in mind, mobile contracts also have a minimum term to run, usually 12 months or more. And the longer you take it out for, the more you get for less money.
You could share a dongle easily but only one at a time.
You *could* daisychain the two devices….might be easy. You would need to connect the two with an ethernet cable, either via a hub or find a crossover cable. Create a network between the two, make the one with the dongle the gateway on the other and then set the one with the dongle up as an internet sharing machine.
Sounds simple enough. Never tried it though. You could also apply variations on this, maybe even wireless between the two. You'd need to know what you were doing though.