In a book I saw the caption "this is Ken and Mike". Should it be "these are Ken and Mike" since it is plural? Or is there a certain rule that I am unaware of? Thanks.
"This (picture) is Ken and Mike." If the sentence is a caption for a picture, "this" is assumed to refer to that picture, which is singular.
However, though it's a hair less idiomatic in the case of a simple caption, one could say "These are Ken and Mike", in which case "these" would be assumed to refer to the individuals in the picture. (Something like "These two people are Ken and Mike" would be more idiomatic.)
If Ken and Mike are considered to be one unit or a pair as in a couple (or: "an item") or partners or best friends forever; or as the two guys I mentioned yesterday or the guys next door then is can be used. Compare this is the Ken and Mike I told you about yesterday.