Vehicle wiring products and others sell an inline fuel tap http://www.vehicle-wiring-products.e...lpumps.php#atv
(no association I'm just a customer), brass T off e-bay. My rear tank is in the previous pillion seat position so when full would be just higher than the main tank. The pipe run goes normal tap-T-carbs with the leg from the T going self sealing QD connector-inline tap- second tank. The inline tap is behind my knee, so easy to get to at low road speed, not so easy when shifting.
I fill both tanks with the taps off then turn on the auxilliary. At 90 miles or so I turn the main tap to on and hopefully the auxilliary off. If I can't reach the auxilliary tap without killing myself and get the switch over point wrong, the levels in the tanks will equalise. This is fine up to about 90 mph at which point the flow is interrupted enough by the siphon effect to get the odd feeling of running out. Not a big problem to me but who knows on another bike it could be a smaller window to change over in and might not just happen at stupid overtaking speeds. By about 150 miles I can just turn both taps on and go as fast as I like, it's just one big volume. Reserve is just like normal and comes up at about 250 miles. Somewhere about the 290 mile mark I'd be walking.
I played with a NRV to try and ease the flow and do away with the two tap set up. They are designed for fuel pump systems and open at about 1 bar against the balance springs and losses. You'd need the tank strapped to your helmet to get enough head to open the valve cleanly. With insufficient pressure the engine would need to pull vacuum to get it to open giving the same issues you are trying to cure due to valve hysterisis.
Don't forget the tank breathers, most MX shops sell little float valve breathers that let air in to stop vapour lock but stop fuel getting out if the tanks do have to level.
Piccies here https://sites.google.com/site/threew...d-improvements
I've had no bother on ferries (the tank is fitted, has a proper cap etc.) but you might with jerry cans. I remove it via the QD and fit the two person seat for MOT day, the pipework would confuse certain Quick Thick fitters as employed by VOSA. Take care if planning on going to Germany they might try legal stuff related to cans in the cities. I've had a petrol station attendant go "official" on me when I opened the second filler cap. He was fine once he saw it was bolted to the bike. Some sort of cover might be in order.