Well, the most famous style using both the
katana and the
wakizashi at once, was that founded by Japan's most iconic
ronin, Miyamoto Musashi. It was originally called
nito ryu ("two-sword school"... how daringly original

), but was later renamed
niten ryu (
Niten being the named Musashi used to sign most of what he wrote). As far as I am aware, that style is extinct (I hear Musashi is known to have had a few students, but I don't think any track of them was kept).
That's about all I know on the matter. I think the regulars of this forum should be able to give you some better information.