You can do it and it works
While it is true that independent redundancy is best, there are also practical considerations:You want a FAT hose – in the case of my 1985 C27, 1.5”. Since the aft end of the bilge is very narrow, there is no practical way to run two hoses of this diameter side by side. Also, clogging may not be your prime concern (since the pump-out would always be through the bilge pump strainer). The real concern is rapid flooding – and a working electrical pump is far more efficient than the hand pump – or failure of the electrical pump, in which case the hand pump would be used.I have both pumps in series – just as you suggest – and it works fine (actually, my boat was rigged that way when I bought her). There was a 1.5” rubber hose installed, probably by the factory, and I noticed that this hose had a tendency to collapse. I replaced it with corrugated plastic pump hose and have no problems since.Good luck,Hans