Minimal method to merge two headphone outputs into one pair of headphones

Hey all!

I’m completely new here, and it’s my first time posting, so I apologise if I’ve done anything wrong.

I currently have two separate headphone outputs, each with their own dedicated volume control knob. I’m looking to merge these two outputs so I can hear them both simultaneously in the same pair of headphones.

I’ve done some research, and discovered that I’m looking for a some sort of mixer. However, all the mixers which I’ve found online are relatively big and all have their own volume control knobs built in. I’m ideally looking for something minimal with no external controls, considering I already have them for the individual headphone outputs. This is being used in a home studio for live broadcasts, which is why I don’t want volume controls on the mixer itself as I do not want it un-intentionally changing and then having to troubleshoot whilst live, working out which knob and at what step in the audio chain that the volume has been changed. Something like a Y-Splitter would be excellent, but in reverse - so instead of one output going to two headphones it’d be two outputs going to one headphone. It is important to me that the audio quality is not affected (as much as possible), though a volume decrease is okay as it can easily be combatted by just increasing the volume of the outputs (which is currently only around 15% anyway).

Does anyone know where I could find what I’m looking for - or even a more specific name to find it? If not, is it even possible to exist, and if so how can I go about looking to get it custom made? I’m from the UK (England), though can do international shipping if it’s the only option.

Many thanks in advance!