Finite number of Turing machines running concurrently on multi-tapes Turing-machine-equivalent?
A roung, informal sketch of how I would prove this:


We can simulate multiple threads running concurrently in a programming language like C. (Note that this is without hardware support! There are some cool ways to do this, my favorite is using continuations in Scheme, inserting a call to a thread scheduler whenever a function returns).
We can simulate a multi-tape TM with a single-tape TM.
We can simulate a single-tape TM in a programming language like C.


From this, we get that we can simulate a multi-tape TM in our programming language. Since we can simulate threads, we can now simulate multiple multi-tape TM's running concurrently.
And, since we can simulate C with a single-tape TM, we can simulate the whole thing with a single-tape TM.

The transitivity of turing-completeness is a useful tool. That is, if A simulates B, and B simulates C, then A simulates C.