Good idea IF you have vacationed with this couple before and had good experiences and you are sure your style of doing Disney matches. Having that friendship at home, doing things together at home, dining at restaurants at home, can be very different when your trip is at WDW and things dont gell. We had a trip that almost ruined a friendship because the other couple wasnt the same on vacation as they were at home. When your styles of hitting the parks and expectations of what you enjoy differ it can create conflict.
As far as FP+... you may have to just accept that your choices may be limited and youll have to endure waits in lines. I have gotten some great last minute FP's but also had been stuck with ones I didnt want.
Restaurants also can be hit or miss depending on the times you are trying to book and availability of tables. Again you may just have to accept seating at a second or third choice spot.
Last minute trips can be just as successful as ones youve long ranged planned. Its Disney after all and if I had a chance to go I'd jump on it and make the best of what I could do.