Jerry,
There are several way to do this.
1) take a line out from one EV and run it INTO the bass amp directly .... roll off the highs and pump up the bass. The overall balance will be controlled by the bass amp knob, but the volume will change in proportion to the EV if the mixer signal changes.

2) Use a dedicated crossover. This will give you the MOST efficient use of power and clarity. Bi-Amping is a great way to increase presence without making the WHOLE signal louder. Here's what you do:
Without a mixer - Run two wires from the keyboard INTO the crossover inputs (L&R). Run the HIGH outputs into the EV's. There will be a summed low output that can be routed to the input of the bass amp. If there is NO sum, then just use a "Y" cord to get the signal from both channels into the single bass amp. You DON'T need two bass amps. One will be fine, because bass is NON directional. The human ear doesn't really know what direction the real low notes are coming from. I'd use a x-over point of about 180Hz. No higher than 250, or you get into the directionality range. Stay with under 200 if you can.

3) WITH a mixer -
If you want to avoid a dedicated x-over - a monitor send can act as a poor man's x-over too. Roll off the bas a little on the KB channel, or if you have a graphic EQ - take it down there. Then use the monitor send to feed the bass amp, and roll off the highs(at the amp's controls).
This method works great because you can add more bass to any signal in different amounts, by turning UP the monitor send on that channel.