In, say, a BN2, if you lose a donk there is only one direction you are going, especially if heavily laden.
Two donks = twice the chance of one failing.
In a Caravan if you lose a (the) donk, there is only one direction you are going.
Personally, I'd trust a PT6 far sooner than a piston donk. Double the number of piston donks and you double the chance of going down.