My split Roubo workbench

[Ron Swanson Jr.]

As long as you havnt glued it you should be able to pound it out by tapping the inside face of the tail board to separate it from the pin board. Even if it's tight it should separate.

[wtnhighlander]

If you can combine a spreader (constant force) with light taps with a moderately heavy hammer or mallet, you'll have an excellent chance of separating it without damage. Either method alone should work, but combining them reduces the shock load on the joint.

[Mark J]

If the above ideas don't work, try this.  Lay the tail board (light colored piece) on a workbench with the pin board (darker) hanging down.  Clamp the tail board in place. 

Now using an appropriately sized dowel tap out the pins from the tails.

Once you get the joint separated, consider if you need to make it slightly looser, given that the wood fibers are going to swell when you put glue on them.