I wouldn't risk it. I would expect that after the 30 days are up, they will charge the extra $50 to your account and then it's too late to send either phone back. The same way that they tack on the extra $50 when people buy the phone on a Prime trial and then cancel Prime at the end of 30 days.You could always initiate a return, and then just not ship the return back, in which case they just charge you the $59.99 after 30 days.... Result, you get 2 phones
The easiest way to get a second phone of the same model is just to give a friend who has Prime the money and have them buy it and ship it directly to you as a gift.