Depends on how you do it and which TV you have. Some TVs do allow simultaneous connections to headphones and main speakers, but most will only do headphones as soon as these are connected. Same thing with line-out; depending on the TV, as soon as you connect something to that, main speakers usually shut down.
From your phone, if you're mirroring the screen, you MIGHT be able to do it (BT through the phone, audio on TV through mirroring, but chances are you will get a very noticeable delay between the two, enough to make this not a good option). If casting through Chromecast, then that's a nope as everything would be 'handled' on the TV (and again, this would depend on the kind of TV you have).