Which model TV do you have specifically?
I'm not aware of too many implementations of what you want - typically, headphones are used in place of the speakers - which is likely how the TV is set up.
Whether an option exists for simultaneous audio "out", will depend on the specific TV..... but I'd be surprised if it did.
There's probably an option to take audio "out" to a 3rd party device (like a soundbar) and then simultaneously out via the bar + headphones, but I'd have to look into it some more.