Can I split 1 set of outputs on my deck to run my sub amp and speaker amp?

It depends what the output is for. Typically if the deck only has one output it is for the sub. You wont be able to run your 4 channel amp on that signal. You will have to run speaker wire from the deck harness to the back and use the high level input (if the amp has one) and then run all new speaker wire to the speakers.

If an HU only has one pair of rca pre outs it's almost always a full range signal.

Using y cables to run signal to more than 2 channels should be fine.

easy just buy a pair of splitter cables