I usually use 4" and 6" CLR or 3" and 5" CLR bends depending on what I can get. The new set of pipes I've built are 4 and 6. I use 2" tube so the 2" difference in the Center Line Radius means they snug up next to each other perfect.
You basically want the difference in the CLR to be as close to the diameter of the tube as possible for a neat stack.
The problem you are going to find is locating a supplier that has dies in the CLR you want. Most are going to be round numbers like 3" 4" 6" CLR etc
I reckon a CLR of 2" and 4" won't look real good. Wider radius bend look better in my opinion.
There are 2" tube with 4" and 6" CLR
These are 3" and 5" CLR in 2" tube
Cheers