To Kknukle,
Sorry the simple answer is no. With the motors you specified it is unlikely tthat you can multiply the nameplate rating.some one might correct me!) The voltage you secified, and the frequency, means that horse power, would be difficult to multipy.
That is unlike battery powered fork lift series dc motors, where many people on this site have used such motors, for example taking a 15 HP mktor and getting mmmm! Maybe 60 HP.
As a point of interest, I am thinking of a custom built lightweight and looking for up to 20 HP continuous and 30 peak to give me acceptable road speed, 50 cruise and 60 mph short time for urban roads.
Have a look on those sites who have used series dc motors, of 24 volts or 48 volts. There is a lot of info on these out there.
Cheers, Joe