We use a single billing system (WHMCS) for our various brands - basically using the products section. Everything gets billed under our corporate name (Hands-on Technologies LLC), but there are divisions to our company - domains, shared hosting, dedicated servers, and ssl. Our SSL is actually a branded Comodo SSL Certificate, but completely separate company.

Depending on investors, issues with accounting, and tracking - you may NEED to keep things separate. It's best to contact your accountat as to how much the sharing of assets can affect your various companies.