To sleep together in the same bed or similar space. Does not imply having sex. It's what "sleep together" would've meant if we hadn't turned it into a slang idiom meaning "to have sex".
"The kids used to cosominate until we got them separate beds when they were 6 and 8."
"You can crash here if you don't mind cosominating. I don't have a spare mattress."
"We just started having sex last week, but we've been cosominating for months."