The Touratech high volume tank on my X Challenge (new to me, it was already done) feeds into the standard tank, I believe without drilling into it. I am sure it feeds into the main tank through the fuel line on a T piece and the standard tank breather is fed to the TT breather on the fuel cap. I fill through the TT cap and it fills the standard tank at the same time, right to the brim, hence requiring the extended breather - you cannot open the fuel filler on the standard tank because the fuel will dump out through it. The two taps on the saddles feed into a single line that joins the standard tank...... when I dig a bit deeper I will find out exactly how it was done (Mickey built the bike, but he can't remember!)