Looking at this now and thinking through a few more things:
I guess if I ripped down the length then removed all glue and jointed the top I could attach the larger half to the bench. I could then work the second part and when it came time to glue up, I could do so directly on the bench to help assure there’s as little movement as possible.
Separate thought- I clamped the two ends with light duty clamps and the gaps easily closed right up. What if I clamped, inserted the dovetail keys and then unclamped? Wouldn’t this keep the joint closed?