speakers and cables

The purpose of having both channel inputs on the sub is so that they can be summed to mono. If you are using the two subs adjacent to your left and right main speakers, you could consider left channel to left sub and right channel to right sub. If they are not adjacent to the mains, you should run both channels to the subs and sum to mono.

Regarding blowing fuses, I suspect the fiddly insertion of all those wires into your power amp resulted in a loose strand that was shorting to the adjacent connector. Have a close look. You might be better to make a y connector, assuming you are using standard speaker wire. A short length of wire, then solder the mains and sub leads pos to pos and neg to neg, using shrink wrap or electrical tape to insulate the connections. There is very little current flow into the high level sub inputs, soyou are not overloading the single y connector wire to the amp. Hope this helps.

@audio_guy_uofw

yes it helps thank you. I have about 5 millimeters between the speaker binders and my speaker wires are thick. I could look for thinner speaker wires. 

I sold my beloved Marantz amp for this reason, impossible speaker binders, swapping speakers was an afternoon long activity. 

I am giving it one last try, hopefully it will leave the fuse alone.

How many splitters would I need for two subs?

You need a splitter for each channel, so two in your case.  I linked AudioQuest splitters but if you check lower down the linked Amazon page, you will find no-name brands for about half the AQ version.

The splitter plugs into your rca output and then you run one rca cable per channel to the amplifier powering your main speakers and one rca cable to the low level input of each powered sub.  This approach would give you stereo imaging for your subs, i.e., one sub for each channel, which is a common approach when running two subs.  In that case I would position one sub somewhat near each main speaker. 

Summing the sub signal is a typical approach used when running only one sub, and sometimes when running a swarm of subs (more that two) throughout the room.  

@mitch2 ok so in dummy terms, red to red, white to white from the preamp?

A single line to each sub?