Physical dimensions first determine what prop size can be accommodated ... then you need to determine thrust level required.
http://www.ecalc.com .... will give you a tool to play with motor / battery / prop combos ... Basically you don't have to fill in all the models data ... just the weight is enough. Then put in battery, ESC , Motor and prop ... you have option to say how many motors etc. Result will tell you if it will work. Note : Just because you have two motors does not necessarily mean half size motors. Thrust is a figure that is needed whther supplied by one or multiple motors - that can be accumulative of motors. But speed is not. You can have 10 motors / props combos all same - but the speed result will be similar to that of one combo - because pitch speed regardless of number of props is a fixed factor. What will happen though is acceleration will be better ... power sharing will lessen load on each motor etc.
I'll fly anything if I can launch it ! Youtube : solentlifeuk