Two base theory is

backed up by absolutely nothing. Every time Goku faces a problem his base form can't handle, such as say, Jiren, do you see him going past his SSJ forms to a version of his base form with a different aura? No. He just goes into his SSJ forms.
Two base theory only exists for those that tout it as impossible for so many secondary characters to compete with post-God Goku/Vegeta such as Piccolo or the U6 fighters (minus Hit), despite the fact that everyone and their mother surpassing the previous peak of power has been a common trend in DB since at least the 22nd TB Arc. Hell, the only reason the notion of it exists is because of the "Saiyan Beyond God" version of Goku meant to represent his base self in FnF, and that only exists for two reasons.
1. Toei weren't fully aware Super would be a thing, thereby thinking Goku had indeed fully absorbed the SSG power into his base and SSJ forms to some extent (the 6/10/15 comparison would make this quite apparent).
2. The creators of DBH probably just wanted a more unique name for the card than just Fukkatsu no F Son Goku.