How to make jumper cables from lifepo4 leisure battery to van battery for if it runs flat while off-grid?

[Hedges]

You don't want anything that would charge LiFePO4 battery without going through BMS, because one cell could run to excessive voltage.
A voltage limited charger feeding BMS should be OK because it can disconnect to protect cells.

If LiFePO4 battery and its BMS feed something that trickle charges lead-acid starting battery, with suitable max voltage, that should be OK.
BMS disconnects if LiFePO4 battery gets too low. Voltage regulated charger protects lead-acid starting battery.

Wires of course would conduct current in either direction according to which is higher voltage.
How do you propose to make constant 1A charger?

There are 12V to 12V battery chargers, like from Victron. That should serve to drain one battery in order to keep other one charged.

 

[OM617YOTA]

https://www.amazon.com/ECO-WORTHY-Portable-Backup-Alligator-Adapter/dp/B017K6PH1S

Maybe something like this. Skip the interconnect to your other battery entirely.