180 amps too much correct?

I view things a little different. If the amps have built in fuses, there is no need for a fused distribution block. Does your large amp have a internal fuse? Even if it does not, a inline fuse that is close to the amp and between the amp and the distribution block is all that is necessary.

As far as the wire run goes, I would use the 2 gauge run only and remove the other line as it is not necessary. Run the 2 gauge to the distribution block and split off with the 4 gauge like you planned. Absolutely nothing wrong with that.

Now for the fuse, the main fuse up front has nothing to do with protecting the amplifiers, it protects the vehicle. This fuse however also needs to be as large or close to the combined value of the two amplifiers fuse values in the rear. Worst case scenario is you need a different type of fuse and fuse holder, be it a circuit breaker type of wafer fuse.

In the future, if you or your son have questions, always post the year / make / model of the vehicle and the proper make / model of the installed or soon to be installed equipment. This way we can accurately map out how to wire a system for you. Heat is what will blow a sub, usually caused by an incorrect gain setting on the amp. The gain setting on the amp is not a volume control. It is designed to be turned down and not up. Answer my above questions and add your new ones to the mix as well, should I still be around, I'll be glad to answer them for you.

1.  You fuse wires according to their gauge.  As long as the fuse doesn't exceed the capacity of the wire it won't be "too much".  Here's a chart to refer to:  power and ground cable specs.  4 ga. wire can be fused up to 150 amps and 2 ga. can be fused up to 225 amps.  The loaded amplifier attached to the wire will dictate the amount of current demand - thus the amount of current that will flow through the wire - and the current flow MUST be less than the wire is capable of handling.  That is to say, make sure the wire is big enough for the task and fuse it to the extent of its value.  That's about it for wire fusing, but there is some good reading HERE on the subject.

2.  To find out what impedance that sub is, first disconnect the wires attached to it.  Determine if it is one coil or two, i.e., one set of terminals or two.  Use a digital multimeter and set it to ohms.  Read the resistance between the set of terminals.  This is DCR and will be slightly less than speaker impedance.  If the reading is around 3.2 ohms, it is a 4 ohm voice coil.  So it the sub has one voice coil reading like that, it's a 4 ohm subwoofer.  Attach the sub to the amp according to the amp instructions.