The KX200.2 wattage is as follows;

50W rms x 2 @ 4 ohm
100W rms x 2 @ 2 ohm
200W rms x 1 @ 4 ohm bridged.

So going from 2 to 4, they all will still receive the same 50W rms that the current pair is getting.

Yes, you can just hang the 2nd pair and wire them to the first pair. Yes, the amp is capable of powering two pair. I would suggest a matching pair though. Same type and size.

Now, is 50W a lot of power to a tower speaker? No, not even for a 6.5". It will do fine for listening at anchor, but may struggle when trying to compete with wind, water and engine noise under way.

If you do decide to upgrade the amp, id stick woth a 2 chnl for the tower speakers. It will get you more wattage for the $$$.

A 5 or 6 chnl is a great option for 2-3 pair of in-boats and a woofer. But would choose the woofer first.