I'm trying to understand what you're wanting to do. I think what your saying is you want to be able to use both tanks by having the front tank drain to the back, or kitchen tank, when it gets full. Mine is plumbed like yours with the number 1 being the bathroom and washer and number 2 is the kitchen. I'm not sure what sizes these tanks are but I know the bathroom/washer tank fills up pretty quick on wash days. I usually just leave both tanks open and let them drain as we use them. If it is getting close to black tank flush time I will close them and let some water build up to flush out the hose.
I don't think I'm helping much but it did take me a while to figure out how the plumbing worked and then I adapted to it. If you come up with something better I'm interested.
2020 F350 Platinum, srw, lbcc, 4X4, 6.7 diesel, 12,400 GVWR
2020 Pinnacle 32RLTS, Reese Goose Box