Can you put two motors of different hp on a small boat like a 7.5 and a 5?

Sure why not, But why?  Backup?

What is the transom rated for HP wise? Will you be running both at the same time or trolling with one? Will you exceed the weight limit on the capacity plate?

Where there is a way there is one are several that will try. Sure why not. But you will not end up with a 12.5 Hp of outboard thrust, more likely a very good push from two fives. The 7.5 will feel drag once you accelerate past the max speed of the five, at least that is my best guess.

I ran a 3.5 hp and 3 hp Eska's at the same time and they worked great together. Just make sure that you don't exceed what your boat can safely handle.

I cant find any 15 or 20 hp motors but I have a friend that has a 5hp so I was wondering if I could try this.

you would not get the additive power of both.  it would probably be a little above the 7.5 hp total output.  

How would you hold both tiller handles?  Would you have to face backwards?

I cant find any 15 or 20 hp motors but I have a friend that has a 5hp so I was wondering if I could try this.

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