Twice as much force?... are you sure?...
The same fixed amount of energy is being imparted by the vehicle so wouldn't each identical strap convert 1/2 the available energy? If the "first strap" converts 100% of the energy, surely then there is no energy left for the second strap?

Different lengths and same rating - the shorter strap reaches its limit before the longer one?... are you sure?...
The manufacturers produce the straps rated to a particular value but in varying lengths so if you had a 9 metre 8,000 Kg and a 12 metre 8,000 KG would they not stretch an equal percentage under the same load and both max out at the same time?. Sure the the 12 metre will stretch further in actual length but why will the shorter one reach its max stretch before the longer one under the same load?

I'm not trying to be a smartrrse also I am not saying I am correct, being the Devils Advocate is a lot of fun actually... I am trying to educate people in the use of recovery gear and "thinking" about the forces involved and the effect on ratings of various components

Here is a direct quote from the 4WD council of Australia safety notes on Snatch Recovery Straps (not winches SNATCH STRAPS) ...
Only connect to correctly rated recovery points on the vehicles, with only
‘Load Rated’ shackles. Load ratings are marked on shackles as WLL (Working Load Limit). Bow
Shackles are suitable for this purpose and should be rated at least 3.25t.

So if indeed the force was double and now potentially 16,000 KG, hows the poor old 3,250 Bow Shackle looking