I may have spotted the problem from what you said, but I may also have mis-read... my comments are in parentheses. Hoepfully, this might give you some other things to examine.
here's the wiring layout: I've got a positive orange lead from the manual switch, a positive brown from the auto switch, and a negative black lead. When I wire the brown to one lead of the float switch and then connect the other float switch lead to the brown lead on the pump and the black lead to the black lead on the pump and actuate the float switch, all is well.
(This is fine, the wiring is done correctly - the auto switch is between the battery positive power and the pump, and the pump's negative wire goes back to the battery. You should have a properly sized fuse between the battery and the switch, close to the battery).
If I then connect the orange lead to the brown lead on the pump and the black lead to the black lead on the pump and run the manual switch, all is well.... until I activate the float switch.
(This MAY be the problem - since you've already hooked up the auto switch and the pump, you should only have to connect the orange wire to the brown lead on the pump, AT or AFTER the connection from the auto switch to the pump, not BEFORE. And did you wire up 2 black wires? You only need one).
Everything runs for a few seconds but the ammeter shows a draw of 15 amps and the breaker blows.
(It sounds like 1-you have two negative wires, 2-a short has been created somehow, blowing the circuit. Also, you should have a properly amped fuse between the battery and the switch, to protect against a fire in the wire - what gauge wire do you have from the battery to the switch? From the switch to the pump? And are all wire connections crimped and heat-shrunk, ABOVE the bilge water?).
So I hooked up a second pump by disconnecting the orange lead from the first pump. I can run two pumps with no problem.
(Not really - if one pump is on "auto" and the other is manual only, you really only have one pump when you're not on the boat. And when you're on the boat, you can only control one pump, not two).
So I'm just going to plumb for running with two pumps. That's better anyway.
But I'm still curious what is going on?
The pump is a Rule 1500 (it only has a black and a brown lead) and the switch is a Rul-a-matic.
(What model Rule-a-matic?)
(Lastly, a blown fuse/circuit breaker could be caused by an over-worked pump - check impeller through the inlet opening to be sure it is not jammed or stuck with debris. Check hose to make sure there's no blockage, and water can flow freely - I usually stick a garden hose nozzle in it, and SLOWLY shoot water into it until it comes out outside the boat.)