Attach two low amp steering diodes (1N4004 to 1N4007) to the solenoid. Anode of one diode to house batt side of the solenoid, anode of the other diode to the engine batt side of the solenoid. Tie the two diode cathodes together (the end with the band), and send that wire to the new "boost" switch. Typically on the dash. Fuse it at the diodes. Put a wire on the other terminal of the boost switch and send it back to the positive control lead on the solenoid. Tie the negative control lead to ground.
When you close the boost switch it will utilize the greater voltage from either set of batteries to close the solenoid. Usually a spring loaded toggle switch is used. You can use a low current push button switch. A typical DPST toggle switch is not recommended since you may leave it "on" and drain the charged set of batteries. Leaving you with two sets of discharged batteries.
My solenoid connects whenever the engine is running. It automatically disconnects the two sets of batteries when gen runs or on shore power, whether engine is running or not. If all else fails, I can manually connect/disconnect via a 500 amp switch.
FWIW Chuck