I am struggling to find a mechanism for the above reaction.
So far I have suggested that the proton adds to one of the reacting aromatic species leaving the resonance stabilized secondary cation. This then adds to the ortho position on the second reactant (since alkyl groups are activating). I then get a positively charged intermediate.
For a simpler reaction I would simply remove the proton but I wonder whether the reaction from this step to the product is concerted? I couldn't see an obvious step so I deprotonated and again could find no obvious way to continue apart from a rather elaborate proton transfer from a carbon that does not possess the most acidic proton.