well that is an excellent question, and I do not know that anyone has systematically studied the optimal order and timing. So I think it comes down to practical considerations, such as “is it even possible to use them at the same time?”
Speculating now, based on how I think PEMF interacts with the body and with other types of devices, I think it might be best to start with PEMF and use it for 15-20 minutes before starting the second device (of any type). I say this because my current thinking is that PEMF works synergistically with almost everything else by reducing the background chronic inflammation. This allows the cells and tissues to respond with the least amount of dysfunction caused by the background inflammation. I get reports from people all the time that suggest this is the case, for almost everything from nutrition to acupuncture to surgery. By this means (I think) PEMF is helpful for almost everything, to make it work as well as it can.
So, taking into account the observation that it takes about 15 to 20 minutes for the effects of PEMF to really start working maximally, and these inflammation-reducing effects seem to be persistent for hours at least, it makes sense to me to begin with PEMF, then follow on with anything else you are planning to use synergistically with PEMF.
The question of whether or not you should continue with the PEMF so that you end up using both at the same time is, I think, more of a practical matter. Basically, my opinion would be to continue using PEMF simultaneously as you add the second device if it is practical to do so.
That is all just speculation on my part, and I could be wrong, but I don’t see any way that the simultaneous use of PEMF with anything would cause a problem. So the worst case in my opinion would be that it is simply just not optimal.
Of course the best information on this will be when people try this and then report back here for detailed discussion.