50/50 is a lie.

For the person working it, both sides often expect priority or "more" than 50%.

When someone wants to 50/50 me, I tell them, 51/49 is fine. But there must be a clear priority when there's a conflict.

Strangely, when I do this, the urge to 50/50 me usually goes away. I have nothing against working 2-4 projects. But there must be clear priorities, or I must be able to set them.

My math is usually that splitting somebody into percentage allocations reduces their total productivity. So 50% + 50% results in 50% total productivity. 30+30+30=30.

The only time this can work if the person knows their work really well and can do it in their sleep. Otherwise the context switches are way too much for most people.

What about 1 week or month, 1 project, 1 week or month another?

50+50% but each week it's clear what the priority is? And fewer task switches

(But I guess most of the time it s better to focus on one & finish it first)

That's probably better. But probably hard to implement.