Would the dpdt make things like grounding easier?

Well, if you have any coax feed, it would allow for grounding the shield, too.

I need to build me another antenna switch array. When I listen to MW and LW, I
have to manually disconnect my feed's shield, which causes attenuation of the
signals from my SW antenna. So my switch box would have at least one switch to
do that.

I usually use DPDT switches, especially those with center off, so I have
flexibility in my final design.

The switch box I use now is left over for when I had a ham rig run through it,
too. It's mostly wasted now, because it needs changed.

I want use a plastic box for SWL-ing. That way I can isolate the coax connector
shells, except when switched together. The shack's ground is a husky ham
station ground per ARRL handbook, copper strap and all.

Vy 73,
Bill, K5BY