Would it be possible to design a macbook that has both an intel x86 processor and an ARM one? Sort of like how we turn off the discrete GPU when not needed. Too expensive or too complex?
It's actually not the cost, though having two functional CPUs would cost more. How would it work? How much time and effort would go into building the hardware so they could interoperate and the OS to make it usable?
Just a massive amount of complexity for very little gain. Apple's Rosetta2 will already run x86 code with very good performance on Apple Silicon, in MacOS as a MacOS app. Hundreds of dollars per Mac cheaper, and far less complex to integrate.
Most likely pick one processor architecture as native for the kernel in, and the other one has just enough kernel to trap system calls and call into the native kernel, and switch tasks.
If, within 6 months of ARM Macs coming out, we have Chrome, the Adobe suite, and Microsoft Office running natively, that will cover the vast majority of Mac use cases.
I guess, like the graphics cards, the "weaker" card is connected to the display, and just passes through information. So with a dual CPU it would really be like virtualization, and the input/output would be handled by the newer ARM cpu, but if they'd need to run an x86-64 program, they'd start up that CPU and whatever info it needs from the OS, there'd be a translation layer.
That card gets you 3 laptop-class quad-core CPUs with integrated graphics.