C.R.S. § 13-25-118

In actions upon express contracts against two or more defendants alleged to have been made or executed by such defendants as partners or joint obligors or payors, proof of the joint liability or partnership of the defendants, or their first names or surnames, shall not in the first instance be required to entitle the plaintiff to judgment, unless such proof is rendered necessary by the filing of pleas denying the execution of such writing, verified by affidavit as required by law.