I actually had to run a splitter off the back of the tube buffer because it only has one output.
So, one goes to my subwoofer amp and the other goes to the main amp.
It may be wrong technically (I don`t know) but it seems to work.
Whether or not that would have a sonic downside would depend on the output impedance of the buffer, the input impedance of the sub, and the input impedance of the main amp. And if either or both pairs of cables that are involved are especially long it may also depend on the the length and the capacitance per unit length of the particular cables.
These things can all be calculated if the relevant parameters are known.
Best regards,
-- Al