Two of the three authors who did the first extensive work on 2-way pushdown automata [1] consider multitape and multihead extensions of 1-way and 2-way deterministic and nondeterministic pushdown automata (PDA).
Michael A. HarrisonÓscar H. Ibarra
Parosh Aziz AbdullaC. AiswaryaMohamed Faouzi Atig