I have a similar setup with a yosepower 350w with pedal assist sensor on rear and a mxus 250w with throttle only on front. It has worked great for the last 3 years and over 20,000km, and has got me comfortably up steep hills that would not of been possible otherwise. The only technical problem has been the odd broken spoke on rear and a noisy wheel bearing on front replaced, and the wearing out of the rims coming back down those hills with v brakes. I will have to change that bike to disk brakes sometime.
Anyhow, I can say that two motors definitely give you double the power in terms of torque and climbing ability. Also, I think it improves reliability in that the motors are less likely to be overloaded and overheated. In fact, the extra motor weight can be an advantage in terms of absorbing heat and two motors can disperse heat better than one larger one.(more surface area)
Of course, some time in the future, I would like to replace these two motors with one larger geared hub (for legal purposes). I reckon I would need something like a high torque 1000w geared hub (e.g. Mac12T) to better my setup.
I have also a little 250w tsdz2 mid drive with throttle and low 34/34 gearing. It will pull me up hills but slowly. Its great for the nearby slow and narrow mtb tracks. However, I find all that gear changing tiring. Also my old legs like a slow and relaxed cadence (50-60rpm) that hub motors are ideal for whereas some of these middrives like lively legs that can spin at 80 rpm.