Is multiprocessing possible on a turing machine?
You can consider two variants of this question.


Can a Turing machine simulate parallelism?
Can a Turing machine simulate parallelism efficiently?


The answer for the first question is a resounding affirmative. Just like your own OS simulates threads on a single-core CPU, so a Turing machine can simulate parallelism.

Regarding the more refined second question, it depends on your definition of efficiency. You can probably simulate parallelism with at most a polynomial blow-up in resources (mainly time).

Turing machines are not a good model if you care about concrete efficiency. The more appropriate model is the RAM machine. RAM machines can probably simulate parallelism pretty efficiently, though you should not accept a running time any less that the combined running times of the individual threads in your parallel solution.