The default/"best" answer is "a Terran and a Klingon", both in the singular.
Various other singular versions are also accepted.
(No mixed versions currently, though I suppose they should be as well. I'll see whether I can add some.)
Note that tera'ngan is a "Terran" -- "human" not the best translation (and has its own Klingon word, Human). Not all human beings necessarily come from Earth, at least not in an era of spaceflight and planetary colonisation.