How to redirect one folder (site) to another in multisite

I need to redirect one folder (site) to another in WordPress multisite. I have a 20 site installation in which every two sites are closely related and they’ve decided to merge.

So thedomain/siteB will need to be redirected to thedomain/siteA.

I tried an .htaccess rule, but nothing happened. But I’ll admit to being far from an expert when it comes to .htaccess.

This is the rule I tried:

RewriteEngine on

RewriteRule ^/?folderA/(.*)$ /folderB/$1 [R,L]

I put it at the end of the .htaccess in the root folder.

Any ideas?