If your chain is A > B and B > C, then no.

You could boost with A, and level BC.
You could boost with C, and level AB.

For every even level gained, the referred (new) account can give one level to a lower level toon on the referring (parent) account.

So, in the above example, C can give levels to B and B can give levels to A.
C cannot directly give levels to A.
But If you were to boost four toons on BC with A.
C can essentially give two toons to B; as B gives two toons to A.
But then B can give one more toon to A, based on the levels received from C.
I believe it is 39 levels maximum, so by full toon (80th, the RAF maximum)... you would level a toon to 2nd or 3rd and then grant it levels...