The way that you want to do this is to run all of your Twisted code in a thread, and make calls into it from a different thread using one of the utility APIs for that purpose.  See, for example, <http://twistedmatrix.com/documents/8.2.0/api/twisted.internet.threads.html#blockingCallFromThread>.  Remember not to touch any Twisted APIs directly from the non-Twisted thread.  Leave the Twisted I/O thread running for the entire lifetime of your process; don't start and stop it repeatedly.

For what it's worth, you should really look into restructuring your application so that this kind of hack is not necessary.  It will almost certainly have hard-to-debug unforseen consequences.  There's nothing fundamentally broken here, but it is much harder to keep track of and get right than you think it will be, just based on the fact that many many people who try this strategy end up giving up on it.