Aut cum scuto aut in scuto
Well, the way I heard it is that the prop was affixed to hide the fact that it was a jet...a military secret at the time. No "dual propulsion".
No, you're thinking about the first jet prototype we flew, the Bell XP-59 Airacomet, and the dummy propeller was only mounted while on the ground and was removed for flight. The CurtissXF15C-1 had both a radial piston engine and a jet engine for Navy testing. Early jet engines had such slow throttle response that carrier operations might be very difficult. However, like most hybrid designs, it was neither one nor the other, and was not successful. Only three prototypes were built. Jet engines improved fast enough that pure jet fighters became viable for carrier operations.
My Dad said he was flying along in a C-119 (which he hated) and a strange plane came up alongside, and the pilot feathered the prop. Plane stayed right there. Turned out to be a Ryan Fireball. Ralph N. Prentiss USAAF.