Existing Limited company - trading as

You will need to adopt one name for trading and form a group however it might just be easier to 'brand' the second idea as part of the existing company if the work likely is loosely associated with your existing business. It would then be a brand name for a service but all accounts and cheques would go into the same account. So for example your business is called Xpro but has Xplus, Xaccounts and Xservices all accounts go off in the name of Xpro for all the other services. However if your main business is IT and your new niche is say painting and decorating you will not be taken seriously and need to open a new account in the new trading name with all that it entails.