If, for some reason you need to run both at the same time (emergency) then yes because you will end up restricting both pumps.
Planning on placing a Y connection at the thru hull?
I see a lose of efficiency with water entering the non running line.
The more I envision it, without some form of valve to prevent water from backing into the other hose, it's a poor design at best but with say a diverter type valve, it might work OK.
http://www.westmarine.com/webapp/wcs/st ... l3wERDpzkAStill seems like a PITA to me but.......