Ahhh... My setup does not allow that option. The aux drains straight into the main tank. If I neglect to close the valve on the aux or open it before I have drained 4.25 ga from the main, I end up with a rough running engine and the smell of raw fuel wafting about. My first experience with that was at 0230 in the middle of the Nevada desert. That was quite a butt clencher!
My first aux setup was on a 2000 R1150GS and I had to count the bars on the fuel gauge before opening the aux valve and turning on the pump. So I know your pain
My BMW and GL are both carbureted so I Tee into the main feed lines and just close the main tank petcock or petcocks(BMW has 2). I just ride until power drops then open the main and feel the surge! I know they run lean for a few seconds but I doubt there is any real harm.
I'm thinking about a GL1800 to I've been giving some thought to its setup.
The only time I've had a fuel smell wafting about while moving was once when I was long into a ride and tired I umm, well, umm forgot to put the aux tank cap back on
Luckily it was wedged on the pillion seat and backrest. No harm, don't think I even lost any fuel. Oh and once when the main fuel line out of the aux about 1in from the outlet developed a crack. Then I smelled fuel at a toll booth and raw fuel drip dripping onto the end of the right exhaust pipe. That will get you heart racing!
OC