I know I had this miscomprehension years ago, and was corrected, so whether it was previously possible or not I don't know, but lever-activation only happens on lever-on 'signal'and lever-deactivation only happens on lever-off 'signal' (from any of several levers attached to any of several leverable items, I think, though I never tried many-to-many, only one-to-many).
The 'solution' to this, I think usually used, is to replace one floodgate with a 1x1 raising bridge (or 1xN, for replacing a row of floodgatez, but raising (either way) on the N-length axis). Lever in one position opens floodgate and raises bridge (floodgate open, bridge blocks) and in the other closes floodgate and lowers bridge (switching the blocker). I presume there'd be some liquid-smashing by the bridge, so you might want to design the system such that smashing happens only upon your decision to open the drain-hole.
An alternate complementary-states (for routing purposes, rather than fluids, so may not be useful for your purpose though it can work) is to have one raising bridge act as a stretch of wall (up blocks, down gives passage) and another lay across a ramp to/from the level below (retracting or raising away from the upper entry, up/retracted gives passage but in built position covers the ramp-hole).