I would guess your problem is one of three things.
1. fuel pump on small motor is bad or weak.
2. The vent on your large tank is partially plugged and the smaller motor does not have enough suction to get fuel.
3. The Anti syphon valve on the fuel tank is to strong to let the small motor suck the fuel out.
Try running the small motor on a seperate fuel system. That will tell you if the problem is in the boat or the motor.
Marine Mechanic
marina owner for 20+years, certified Evinrude tech
That is hard to believe. If it runs on the other tank then the problem is in the boat not the motor. I know of no other fuel pump available. Did you check the vent on the large tank?