Installing two electric motors together with one chain will result in an increase in power equal to the combined power of the two motors.
Although I recommend to use two identical motors when doing this, in your case the two motors in your photo are very similar in power and RPM so I think they will work fine together. The two motors will need to have the same Voltage rating which I am fairly certain that in your case they do.
This two electric motors on one scooter modification has been done by our customers many times with great results.
Two motors would probably run the batteries down twice as fast as one motor would if the scooter was ran at full power most of the time. If you make a switchable 24/36 Volt battery pack the Voltage could be switched when the scooter was moving, I would recommend to let off the throttle for a second when switching the Voltage though.