Yeah, that's the problem. It'd be great as "they complement each other," except that the baseline for Solars is "the best at everything they bother to be try to be good at, given significantly less practice time than anyone else would take." If the Lunar can fill in for and complement the gaps in the Solar's abilities, it's because either you need to do things in two places at once (in which case the Solar has great mortal-empowering abilities and Sorcery to fill in about as well as the Lunar does) or the Solar hasn't bothered to learn whatever it is (in which case what they Lunar's doing probably isn't a big deal).

There's a bit more space for it at low-Essence when the Solar hasn't had the time or XP to be the best at everything, but eventually they'll just cap it all.