I own a mac mini, which only has minijack for sound output from the computer to whatever sources you have. So, I bought 2 studio monitors and I am using RCA to MINIJACK cables from each monitor. How do I route them together into 1 minijack connector and not lose any performance or gain on either monitor?
The mini-jack will be a stereo jack socket, which requires a TIP/RING/SLEEVE plug to interface correctly.
The two signals you will need are TIP/SLEEVE and RING/SLEEVE - with SLEEVE being the signal ground. You should be able to obtain a stereo minijack to RCA converter cable or plug adapter that will do this for you. The signal loss across this connection won't be noticeable.
Either get an appropriate adapter/connector from your local electronic retailer or you should be able to solder one up yourself.