Good question and make sure to talk to your accountant about it.
An initial response off the top of my head...
Advantages of just one company
1) One company is simpler to run from a paperwork and tax point of view.
2) If New Shop loses money, they can be set against the profits of the Existing Shop. It's likely to lose money initially at least.
3) The landlord of the new company will probably be happier to give a lease to an existing trading company with a record
4) Creditors will be happier to supply goods on credit to an existing business with a record of paying them on time.(This might not be that much of an issue as even if you set up a new company, the existing company can buy the stuff and sell it to the new company.)
Advantages of two companies
1) If the new company bombs completely, it won't take down the existing profitable business
2) If a member of the public or an employee sues the company, they will be suing a company with less assets
3) If you want to sell one part of your company, it's easier to sell one company than the assets of a company
4) If you want to give the manager of one of the shops a stake in the shop he is working in, it's easier to do with one company.
I have seen a healthy business in danger of being brought down by one very poorly performing store with an onerous lease. Had the onerous lease been in a separate company, it would have been easy to close it down.
