I just had a brain flash.
A few days ago, I siphoned fuel from the teardrop tank to both engines' tanks. Of course I had to suck on the hose to start the siphoning.
That might be why there is no siphoning effect.
I might have to gravity-feed both tanks individually by keeping the teardrop tank above the engines when it's time to fill them.
Or instal a fuel pump under the teardrop tank.