Yes, Burbank used multiple species for his work on plum selection. For example:
Santa Rosa.—1906. A complex hybrid containing a mixture of Prunus triflora, P. Simonii, and P. americana, with the triflora characters predominating.
Beauty.—1911. “The product of a very complicated heredity including several species.”
Formosa (Wickson Challenge).—1907. A mixture of triflora species and several others.
Gaviota.—1900. Prunus triflora x P. americana.
Lieb.—1914. “Of complex heritage. Among its ancestors are the Burbank and the Satsuma. But there are numerous other strains represented in its heredity.”
P. triflora is the old name of P. salicina.