Agent Scully, Agent Mulder (The X-Files). Occasionally the crossovers here at The Hero of Three Faces posit that the two sets of characters aren't on separate fiction planes (especially when, like this cartoon, the meeting is a holdover from my first fanfiction site where the crossover paradigm was that everyone lived on one plane). But it may be that the two planes' relationship enabling the meeting is only temporary even when there isn't an explanation extrapolable from the sourceworks. We can only assume that this meeting falls into that category because I've always meant to write a followup to this one but I never have.