I'd either copitch or pitch the expressive yeast first. If you pitch it later, then you may get very little character out of the expressive yeast because it isn't growing, it may be less active, and some of the substrates that it uses to produce flavor compounds may have already been used up by the primary yeast.
For example, WL Sacch Trois (the formerly incorrectly-labeled Brett Trois), does absolutely nothing when added as a secondary yeast. Also, I have used a couple of English ale strains as a copitch with US-05. None of those beers have had any esters that I normally associate with those strains when used alone. Some of that may be from overpitching, but I think some of that is also from the possible causes I mentioned above.