Combine or Isolate flat battery for emergency start?
In a 2-battery setup (one 'start' battery, one 'house' battery),
if the 'start' battery is too flat to crank the engine, and you want to use the 'house' battery to start the engine, is it best to:
a) combine (parallel) up the two batteries
b) isolate the flat 'start' battery and use only the 'house' battery to start the engine?
I'm thinking that :
combining one flat(ish) battery and one full battery may offer more current for cranking (the combined battery internal resistance is lower when paralleled) - so (a) is better,
the combined battery voltage will be lower, so the starter doesn't spin as fast...so (b) is better...?
(I'm imagining the scenario where an engine problem has led to excessive cranking and flattening of the 'start' battery, as opposed to a problem with the 'start' battery itself causing it to be flat)
Is there an actual answer, or is it more of a 'depends on how flat...' kind of answer?