can a co-owner of a trademark be sued for infringement by another co-owner

This question is overly general and therefore difficult to answer. The general answer is that if someone is co-owner of a trademark they have equal right and access to the trademark as the other co-owner. That means they cannot be sued for infringement. However, there may be an agreement between the parties over the use of the mark that the one co-owner's use violates. Also, there may be an unfair competition claim. Many times, even though there is a Federal trademark law, theses issues are really State commercial law issues, so you shold contact a local qualified attorney in your area for a free consultation.

Oscar rightly notes that the answer to your question depends on facts that you have not disclosed -- and likely cannot, and definitely should not, disclose in a public forum.

I write to note that the premise of your question assumes there can properly be two "owners" of the same trademark. The situations where two persons can own the same trademark (as used on the same goods and services) are few and far between. "Joint Owners" and "Concurrent Users" of the same trademark do exist but only under very particular circumstances -- normally by a very specific written agreement but sometimes by virtue of the two persons' geographic seperateness.

Even assuming that two persons agree (in writing) as to the ownership of a trademark, that agreement must still satisfy certain trademark rules -- and if the agreement does not satisfy those rules then the "agreement" is simply void. To answer your question, then, a trademark attorney must first evaluate the two parties' claims of ownership. If you're asking the question for purposes of dealing with a real-world situation then you need to discuss the facts with a trademark lawyer.

As I understand your basic question, it is can the co-owner of a trademark sue the other owner for infringement of the mark. While certainly this is fact specific, as the other two answering attorneys point out, the basic question seems clear. The trademark statute itself bars actions against the registrant of a mark for common law claims or claims of dilution or damage to distinctiveness or reputation of the mark. See 15 U.S.C. § 1125(c)(6) (link to statute below). In addition, I am aware of at least one district court opinion in Michigan, Derminer v. Kramer, 406 F. Supp. 2d 756, 757 ( E.D. Mich. 2005), that has specifically found that a "co-owner" of a trademark is not permitted to file an action for dilution against a fellow owner under the plain meaning of 15 U.S.C. § 1125(c). This court focuses on the distinction between the phrase "any person" and "owner" in the Trademark Statute and came to the conclusion than an action, whether for infringement or dilution, can only be brought against "any person" other than the owner. This reasoning certainly supports the theory that a co-owner is not entitled to bring an infringement action against a fellow-owner. And, as you point out, further support is found in the context of co-ownership of a musical composition, another form of intellectual property, wherein you cannot bring an infringement action against a fellow owner of a musical composition. See, e.g., Thompkins v. Lil' Joe Records, Inc., 476 F.3d 1294. In the case of musical compostions, co-owners each effectively own 100% of the musical composition, much like the concept tenancy by the entirety in real property. So, while the facts of your case most certainly should be analyzed by appropriate legal counsel and you should not rely solely on the advise of me or any other lawyer on this forum, there is a very likely chance that your action would fall in contract and tort claims and not under the Lanham Act.