Loops files use Rosetta numbering, which is continuous throughout the full structure. So there is a unique numeric identifier for each position. You need to create a mapping from the individual chains, in order, to Rosetta numbering.
For your complex scenario, it sounds like you’d want a loop file with multiple loops (see example below). Then both loops will be conformationally sampled.
LOOP 23 30 26
LOOP 123 130 126