No need to go to a store... you can call customer service (with both of you available to verify your separate accounts) and they can do it for you.
Or, if you have a 3rd unused phone to assist the juggling process, you can do it yourselves online or by dialing *228. The third phone would act as a placeholder... you'd need to take one of the phones off of one of the lines first before being able to activate it on the other line. So first you'd activate the 3rd phone, put that original phone on the friends line, then friends phone will be free to activate on your line... easy peasy!
Only word of caution, activating phones on other accounts cancels warranty through VZW, but should still be good from manufacturer.