I want to run more than 4 speakers off my stereo system, I currently have 1 - 2channel amp powering two subs and 1 4 channel amp powering 4 6" round speakers.

What is needed to run more 6" speakers including tweeters off 1 head unit? I see all these boats with 8-10+ speakers in them, how do they power all those speakers without having Ohm issues?

They can get "creative" with series and parallel wiring of the speakers.
here's a calculator to show you:
https://soundcertified.com/speaker-ohms-calculator/

Yep it can get complicated for sure. I need to do my homework on this.

One way is to have a separate 2-channel amp for each pair of speaker, and run Y RCA cables to multiply a channel set, e.g. you want to run (4) speakers off the front channel, and (4) speakers off the mid channel, and (4) speakers off the rear channel, and have your subs run off the sub channel...you'd run a Y cable for each channel to both amps off the head unit. This gives you a lot of flexibility. Or run 4 channel amps off each of the FRONT, MID, and REAR channels from the H/U then make your speaker wires run to your various speaker...this gives you two pairs of speakers running off ONE head unit channel. Modern amps and head units allow you to set the crossover frequency with near infinite adjustment. There are many, many ways to run multiple sets of speakers from ONE head unit as long as you are using separate amplifiers. If just trying to use the head unit's output of ~25 watts per channel, you'll need to select your speakers carefully for ohm rating (impedance) and use either parallel, parallel/series, or series speaker wiring, in order to maintain the ohm rating output of the head unit. Wally has a good link there.

Leave the 2 channel powering the subs.

Adding 4 more speakers may be as simple as just wiring each one of them to each one of the existing speakers. It all depends on if your amp is 2 ohm stable. Running component speakers with separate tweeters should not make a difference

This is exactly what I was wondering, can I split the RCA's to add 2 amps off each RCA pair. So my head unit has a front channel, rear channel and sub their is no Mid channel set of RCA's.

Does my simple sketch meet what your saying?

Yes...done that plenty of times...no issues ive run across yet

That's my plan then, thanks guys.

Just don't run the RCA's parallel to the power wires to avoid any electrical noise coming through the amps

Most amplifiers have rca outputs. Instead of all the splitters you could just hook the RCA out of the #1 Front speaker amp to the input of the #2 front speaker amp. So on and so forth.