Yes, I think it's a possible to mod the OEM filler. I have one here if you want a core, you could then pass yours on as a core to others.
In the comparison photo, new is silver, OEM single tank is black.
The changes would be:
Add a sub tank fill bung
Add a sub tank vent line behind the cap (or tee into the current vent line, which would be way easier)
Cut off the upper vent line used previously by the charcoal canister (it vents to atmosphere, not behind the cap). I guess this is optional.
I wasn't able to get a combined gauge solution without involving a raspberry pi or similar, which IMO introduces too much failure points. The manual switch gauges work fine. If I have a reading of anything other than F, that means I'm counting down on the main tank. If I'm curious what the other tank has in it (gauge reads F), I can switch over and see, then switch back. In practice, I always 'know' generally anyhow, but verification of the actual level in sub tank is nice in that rare instance it matters.
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