D.C. Code § 14-304

Where any of the original parties to a contract or transaction which is the subject of investigation are partners or other joint contractors, or jointly entitled or liable, and some of them have died or become incapable of testifying, any others with whom the contract or transaction was personally made or had, or in whose presence or with whose privity it was made or had, or admissions in relation to the same were made, are not, nor is the adverse party, incompetent to testify because some of the parties or joint contractors, or those jointly entitled or liable, have died or become incapable of testifying.