Can I use metal knockout for additional NM cable?
You have to use something when ever NM cable enters a metal box or enclosure to protect the cable from abrasion.
If you use a clamp for two cables, it must be rated for two cables of the applicable size.
For example the Arlington NM94 is made for a 1/2&quot; trade size knockout and can be used with one or two 14/2 or 12/2 NM cables.