True, but that isn't what was suggested. The post I was responding to suggested that it was probably one RH-rotation prop and one LH-rotation prop, with the latter therefore rotating "backwards". In this situation the RH prop would develop full thrust while the other would develop ~50% thrust in the opposite direction, giving a total of 25% of the *expected* thrust.
If you had two identical RH props with one being fitted backwards then your total thrust would be in the region of ~75% of the expected value (for any given throttle setting), but while I suspect that may be what actually happened it wasn't the scenario I was responding to.
OTOH if you had one RH prop fitted correctly, and a LH prop fitted back-to-front (so that it could rotate "forwards") bothg props would produce 100% of the expected thrust but in opposite directions, giving a net thrust of zero*
PDR
* Except that they wouldn't - the rearmost prop would probably produce slightly less thrust due to operating in the wash of the prop in front of it (the amount of reduction depending largely on the rotational speed and the solidity of the props) so there would be a (very) small net positive thrust