First, as a matter of law, a business that is operated under an assumed name, but which is not formed as a corporation
(INC), general partnership
, limited liability partnership
), limited partnership
(LP), or limited liability company (LLC
), is not a separate legal entity. Rather, it is a "sole proprietorship," operated by its owner, and as such is exactly equivalent to the owner operating as an individual, but under an alias name.
If you are running the current business as a sole propietorship, then you can report as many businesses as you like under the same EIN (or, even under your own social security nuber), as long as you can adequately account for each, in the event of an audit.
However, if you are operating your current business as a general partnership with your spouse, then that business needs a separate EIN, and unless you want your new business enterprises to be part of the same general partnership, then you would need a separate EIN for each of the new businesses, or, at least one new EIN under which you would operate those businesses separately from your current general partnership with your spouse.
Re registering different assumed names, the answer depends on whether or not you need those names to do business. If yes, then you must register them, otherwise, for example, you cannot open separate checking accounts for each business.
I hope this clarifies things for you. If not, feel free to ask a follow-up question.
Thanks and good luck!