Unless ya'll are both small folks,move up to at least a 16' with a 17' being even better.
I think 16' is the way to go, especially if you are both fishing. In a 15'er or smaller, the bow fisherman must be very careful with backcasts.
If you get a 16', I recommend a symmetrical boat with the bow seat designed so you can reverse the boat and paddle solo from the bow seat, with the boat backwards. Many 17'ers - in standard, all-around recreational configurations - will be challenging to paddle solo.
I currently own a Dagger Legend 16, which is an all-round boat but definitely more of a river design than a lake boat. In hindsight I wish I'd purchase a MR Explorer 16, as it would be slightly better on flatwater. Now, with three kids and a dog, I feel the need for a 17' in one of all-around designs, like the Camper or Tripper.
Good luck!
