Yesh Omrim (second opinion in Shulchan Arukh)

Are two korot placed side by side valid if neither is wide enough alone to hold a half-brick?

The two korot must be within one tefach of each other to be considered a single valid korah.

What is the minimum width of a mavoi requiring a corrective arrangement?

Any mavoi less than four tefachim wide requires no corrective arrangement.