That's a tough call.
If the total separation is a small fraction of the total length of the seam I would be tempted to just try working some glue into each separated area and watching it carefully next season.
The basic technique for that is to spread the separated area as much as possible (ex. with a thin putty knife) and get glue as far into the gap as possible. I've used brushes, a putty-knife, and thin syringe needles to try and get glue well into minor separations.
I've also been involved with splitting a number of components (springboards, plank, and mast) when they had really bad separations, or when we weren't sure what the interior of the part looked like (ex. rot). It's not that hard (although a Renegade plank is pretty wide).
One problem is that any way you cut the plank apart will remove some thickness so when you glue it back together it will be softer.
This may be good if the plank is too stiff.
It might be possible to replace the wood removed by the kerf with a new (thin) layer when you re-glue the halves together.
Another option is to selectively cut where there is separation and glue in a replacement piece. You could "plunge" a circular-saw blade in and then fill with a half-moon shaped piece of wood.
Hopefully others will chime in with more suggestions...
Cheers,
Geoff S.
