I fear this will only lead you into some serious problems. For one, you could cause significant cavitation (bubbles that come out of solution under friction, pressure and turbulence) in your impeller and this could lead to failure of the impeller.
Second, the pressure variences could lead to an overpressurization of one filter over the other. This could lead to leaks, and ultimately failure of one or both filters.
Three, if one pump fails for any reason, the other may not have the fortitude to run both, again leading to further failure.