A Combi-Micromaster combines with another Micromaster to form a larger robot. They seem to be the logical inversion of Micromaster Combiners, wherein two Micromasters combine when transforming to form a larger vehicle.
Combining in pairs to form a larger robot is part of the Multiforce's schtick, but that team was never identified by this name.
Note: Combi-Micromasters were mentioned in a UK Letters Page, but no specific Transformers have ever been identified as such. Multiforce were labeled as "Micromasters" with the 2004 reissue of their toys, retroactively making them candidates for the name.