I need two displays often so I would but whatever works for you. Having said that I’ve been surprised how much I can get done on the iMac 24 split screen due to the resolution. To be honest though once you get used to spaces on Mac OS you can be productive even on an Air 11 or MacBook 12.
The Macboook would be a supplementary display in that configuration, you wouldn't look at it often. I really liked that macbook design too, I have a feeling the next Macbook Air is going to use that design with an updated keyboard.
Actually better for your posture too (lol) not 100% but that looks like a top end Dell monitor as well that on their own cost iMac 24 prices! The fact that there is an MX master combo there makes that probable (lol) MacBook 12 inch? One of my favourite macs that everyone else hates! The conflict is real (lol) You’d definitely be better putting that on laptop stand to the left and getting a MX keys mini as your back/neck will thank you long term!
Can also pivot/rotate unlike iMac