There's a
NACA paper on propeller drag which compares (table on the penultimate page) the drag of:
1. A stopped, fully fine prop
2. A stopped feathered prop
3. A "free-wheeling" fine prop
4. A fine prop driving a dead engine
5. A fine prop in front of a running engine at idle
At high speeds (just 100 mph for the large prop they tested), the feathered prop does best by a huge margin. There is little difference between 1, 4 and 5, and the freewheeling prop 3 offers about half the drag of those if you can't feather, though it is of course not an option in most aircraft.
At lower speeds, the idling engine produces some thrust, but otherwise there isn't much difference from the higher speed case.