A system is provided for coupling individual fenestration units together and sealing the gaps between them so as to form a sealed compound fenestration unit. The individual fenestration units include mating channels and tabs, or other connectors, that are attached to outside frame surfaces of the individual fenestration units. When the connectors are coupled together, they attach the frames of the individual fenestration units securely together in such a way that gaps are formed between the units along their joined frame surfaces. The gaps are sealed by resilient sealing strips that are configured to be inserted into the gaps, and that are formed with seals that bear against the walls of the gaps to form impervious moisture seals.