It is possible to have two or more businesses registered under the same ABN. However, you must make sure they have the same business structure. If the business is a different structure then you will need to apply for a new ABN.
Our recommendation generally is to have one main/parent company that is your S Corp and that main/parent company would then own your other entities (whether DBAs or LLCs) so that all activity would eventually flow to your S Corp which would then flow to you. This allows for only needing to file one S Corp tax return.
You can create separate LLCs or corporations for each of your businesses, because there's no limit to how many a person can form. The biggest advantage to this approach is that each business won't have to assume the risk of the others; they'll all be legally and financially protected from one another.
LLCs can have an unlimited number of members; S corps can have no more than 100 shareholders (owners).
Mergers combine two separate businesses into a single new legal entity. True mergers are uncommon because it's rare for two equal companies to mutually benefit from combining resources and staff, including their CEOs.
The C corporation is the standard (or default) corporation under IRS rules. The S corporation is a corporation that has elected a special tax status with the IRS and therefore has some tax advantages. Both business structures get their names from the parts of the Internal Revenue Code that they are taxed under.
Because of the one-class-of-stock restriction, an S corporation cannot allocate losses or income to specific shareholders. Allocation of income and loss is governed by stock ownership, unlike partnerships or LLCs taxed as partnerships where the allocation can be set in the partnership agreement or operating agreement.
Our recommendation generally is to have one main/parent company that is your S Corp and that main/parent company would then own your other entities (whether DBAs or LLCs) so that all activity would eventually flow to your S Corp which would then flow to you. This allows for only needing to file one S Corp tax return.