if you use an rca splitter than yes both amps on the other end of the splitter will be controlled by that one output. each end will only see half the voltage of the output. if you're hooking up to a mono block then you only need to plug the one rca cable into either the left or right input on the amp. those left and right inputs on your amp are wired parallel internally so it literally makes no difference if there's one or two rca plugs going into the amp. In fact if you're amp has rca outputs for daisy chaining another amp, those "outputs" actually are just terminals always wired to the same spot your inputs are going to. using my LOC from my stock speakers i can get about 7v rms out of my maxxlink per output without clipping. a 7v preamp signal is more than enough to get the job done for one amp.
one of my troubles at first was when i hooked up my 2 rca plus is i didnt realize that the outputs on the maxxlink were not left and right but rather - and +. this is for strapped amps only... so when i plugged my left and right rcas into the + and - on output A the two opposite polarity signals canceled each other out and my sub was barely moving.
hope this helps