This is also called "redaction criticism", it's widely debated amongst Christian circles. The hypothetical common source for the common content of the gospels is referred to as "Q" as one of the other posts mentioned. The theory suggests that there may have been a document of common sayings attributed to Christ circulating before the formalized gospels from which the writers drew some of their parallel stories.
I don't necessarily think that it has anything to do with Q+, but I'm reminded sometimes when reading posts by Q that it is likely a team effort with multiple people (at least 2 individuals by my estimation). I've always likened my subtle recognition of different individuals posting as Q to redaction criticism. I'm glad that others have made some kind of connection to that.