How do I make this space work?

[DoingItUp]

This is my living room/dining room (photos shortly after moving in so please excuse the mess!) and I’m struggling to know how to decorate it. I really dislike the space and avoid spending any time in it, partly because it’s so cold and the open plan means it’s difficult to heat. It’s also an awkward space being over three levels with some quirky features.

I plan to square off the arch and install double doors with glass to keep the open plan feel but separate the rooms to make it easier to heat.

In the dining room, I have an idea to use teal and gold or mustard with panelling and feature wallpaper. The living room I’d like to be lighter, maybe duck egg colours (I like Dulux tranquil dawn). I’m not sure how to make both these schemes work together though. Will it work to have one side dark and the other light? How can I tie them together? I’d really appreciate any pointers as I’ve been thinking about this for two years and still can’t decide! TIA.

I think doors are a great idea but I would swap the rooms around. The room with the wooden floor seems to have a fireplace. I can't see if there is a wood burner.
The room with carpet is never going to be cosy as it has stairs in it. I would think about putting some glazing into the window recess onto the stairs to help with heat retention.

Thank you @Yewtown. There is a log burner in the fireplace but we don’t really use it. I have considered swapping the rooms around but the kitchen is behind the dining room and it feels more like a dining room iyswim, I think because the ceilings are higher in there.

My plan is to install a faux fireplace with an electric fire in the living room and I’ve thought about running curtain tracks along the ceiling to sort of seal it off when it’s very cold. I have wondered what to do with the ‘window’ on the stair case, one idea was to make it into a fish tank but it would be really difficult to clean being so tall and narrow. Glass could work.

My main issue is working out what colours to use to keep the spaces separate but connected. I’ve found a wallpaper that comes in both teal and duck egg but I’m not sure the difference in light vs dark would work. Do teal and duck egg work together? Perhaps with an accent colour to provide some continuity.

The doors will make them feel like separate rooms. I have teal in my dining room which is open to a snug with cream and copper walls.