iMac internal speakers and external speaker simultaneous

Dear All,

Is there any way to play iTunes radio through the internal speakers and also have a bluetooth or some sort of USB speaker enabled through the headphones or USB outputs so that both the internal speakers and an external speaker could broadcast simultaneously?

I want to play iTunes radio through the iMac speakers but have another speaker on the other side of the room to block out noise coming through an open window -- the motorcycles seem to be rumbling all summer because of a major intersection nearby.

Do the internal speakers have to be disconnected before an external bluetooth or other audio output/headphone connected speaker would work? I think the answer is no, but thought I'd check. An older post suggested using the utilities midi setup to set up internal and external, but I don't know if that will work for the latest iMac -- it also said there was a slight delay in the audio to the external speaker, and that would not be acceptable. Thanks.

I'm working with the iMac 5k with retina display, El Capitan 10.11.4.