I'm pretty sure that this would not work as you envisage and that macros will not help. Group 2's fixture would need to be controlled by both sets of faders and would show whatever level of each colour was set last. Other than long and complex macros that add and remove group 2 in group 1's faders there seems to be no way out of this.
A far simpler solution might be to arrange your faders by colour rather than group...
Instead of G1-R, G1-G, G1-B, G2-R, G2-G, G2-B
Use G1-R, G2-R, G1-G, G2-G, G1-B, G2-B
That way, when you want 1 & 2 the same you use the pairs of colour faders together at the same level.
PS I have an FLX and use a midi keypad to set all LEDs to one of 13 colour pallettes but I can also set the even number fixtures to a different pallette. This is not possible without the midi inputs of course. It takes 28 buttons & 30 playbacks on pages that are not current to do it but it works pretty well.
If or when OSC becomes available on S24/S48 you could PM me and I'll explain it. Would take too long here.