There are plusses and minuses for both.
With two motors you have the option of controlling them independently so you can do more interesting manoeuvreing and get involved with mixing for more interesting turns etc. Plus if you supply them with a seperate battery, as I have done with my Deans Marine Cossack,you will always be able to get her home if one side fails. The down side is the the two could well never be exactly in synch as they will have thier own speed controllers and you will need an additional channel on the radio and another speed controller.
One motor has the advantage of always being perfectly synchronised but the shafts, of course, then have to work together. They only need one speed controller and one radio channel. The disadvantages are that they will have to use a gearbox, with it's inherent losses and noise generation, and if anything goes wrong with the speed controller you will have to recover the boat.
At the end of the day there are no right and wrongs it is entirely up to you to choose what you prefer to do.
I hope that this helps.