Sorry I was unclear about this.

I would only joint two boards together to make a single "plank".
I would make two "planks" and leave a space between them.

The only surface that must be flat faces up.
Rather than make one large top, I would make two
smaller pieces and mount them to your trestle.

I was under the mistaken impression that you intended to rip these
lengthwise, and turn them 90 degrees to orient the "sides" upward.

Sorry if I muddied the water.

******

If it was me (and it's not) I wouldn't even bother with flattening the bottom of the planks.
Once you make the trestle or base, the only part that needs to be flat is where
the planks rest on their supports.