Working between two networks

[TriedandTested]

We have a member of staff here with a laptop..she works between two sites a lot. Her laptop is currently set up to operate on one network, but i would like her to be able to simply come to the other site, plug in and access this network. I understand that setting up trusts between the servers is one way of doing it, but i dont have much experience doing that and was hoping there was an easier way or this that the only option?

I basically want her to have the options of the two domains on the login screen.

Thanks.