Using two headphones at once with UR44 and more

I want to replace my old Audio Kontrol 1, it sounds great but the headphone jacks are really poop, I have to wiggle the cable to make it sound right, something is broken.

Anyway I wanted the UR44 cause I heard it has better drivers than the Focusrite 6i6 which was the other alternative I was considering which has 2 headphone ins.

I want two so I can have one for mixing, viewing a movie, and other sound sensitive stuff then some cheaper earbuds for youtube, and non sensitive stuff, so I dont have to wear my big beyerdynamics all day which give me an headache.

My question is if I can have both at once, and then mute the one im not using, instead of having to lower the hardware knob for the volume of the phone. The reason is I want to have a fixed max volume on the hardware knobs which I never need to touch. I hate having to adjust the knob everytime I go from headphones to my earbuds, so with 2 separate knobs I can have it all fixed the same every time, then, just mute the one im not using, but im not sure if you can do this. I saw this pic here:

I see on the lower right, PHONES with the green 1 and 2 icons, so I guess he has two at once connected. However, does this mean that only the one that has a green square (the 2 one) is sounding, and so the 1 is muted? how does this work in practice? please let me know.

I also want to connect my Adam monitors in there. I connected them in “1 and 2” outputs in my AK1:

So if im not mistaken I must put my monitor cables in “MAIN OUTPUT” here:

What are the “LINE OUTPUT” ones for? to connect another pair of monitors or other things? In my AK1 I never understood why buy you can connect 2 pairs of monitor at the same time, it even has 2 separate knobs at the front:

See “Output 1/2 and 3/4”. I never understood why the hell would you need two pairs at once but whatever.

I assume that the knob that controls the monitors is the “OUTPUT 1” in the UR44

What are all these “Line input” knobs doing? there are 4 of them. But I only see 2 “Line inputs” to put 2 cables.

Im very confused because I grew up in the VST and software era and I have never used any hardware beside my AK1 to plug my headphones.

In the future I want to buy some hardware, maybe something that is a complete package like an Eventide H9000. Can I plug it there?

Also my headphones are the DT770 600ohm, I assume this has enough power to drive them. On m AK1 they sound as loud as needed and very crisp.