Can we both have our own corporations in the same business?

You use the term "business partner." Does this mean that you have a Partnership Agreement? How do you divide up profits or losses? Is that written down? Did you have an attorney draft the Partnership Agreement? How do you report your income on your tax returns? You also write that he "changed our corporation name." Are you a corporation, not a partnership? Who owns the stock of your corporation? Are you an LLC? A multi-member LLC is taxed either as a corporation or a partnership. If a partnership, there needs to be a partnership agreement. The question you ask is a relatively small issue, I think. The answer is yes, two corporations can enter into a Partnership Agreement, just as two individuals can. The bigger issue is what is your "partner" trying to accomplish by "incorporating himself only?" Has he changed any bank account signature authorizations? Who keeps the books? Do you still have access to them? Is your "partner" really trying to cut you out of the business? You should see an attorney immediately to discuss these and perhaps many more questions. The attorney should not be one whom your partner uses or the attorney who has had anything to do with your business.

It is certainly possible for two or more corporations to be partners in a partnership. I know of several partnership where one or more of the partners are corporations, including some where all the partners are corporations.

You could each have your corporation own your stock. I seen no real benefit to this and it could cause potential problems with taxes. You could amend the Articles of Incorporation to change the name and then each own an equal number of shares. Technically, this would be a "reorganization" under Section 368 of the Internal Revenue Code but should not have any adverse consequences since the only thing being changed would be the name of the corporation.

It is possible for both of you to have corporations in the same line of business. I would need more information from you to determine whether his formation was a breach of his fiduciary duty to you.