I assume this passthrough mode somehow makes wiki entries compatible to be displayed in a WP loop? If not, there's little hope of combining the two results. Even so, you probably still have two separate results from two separate queries from the single search terms submit. Unless you can somehow concoct one grand all encompassing query that returns all results in a WP loop compatible form, I don't see how the results can combined without significant post query processing.
So if I understand things correctly, maybe you have a special search results template with two columns. One side is basically the standard WP search results template. It will load during a normal WP search. Then the other side will grab the search terms used and access the wiki search api, then display the results in it's own column.
As alluded to above, you could post process the two results into a single combined list, assuming they are compatible, but I'm not sure it's worth the extra processing.
I doubt I totally understand your circumstance, so my approach may not be exactly right, but hopefully it gives you some idea of a possible approach.