I need an explanation about an MF build

At minute 0:29 of this video, he shows two Crystal Sword secondary weapons with 6 Ist attached to each one, but in gameplay he always has his primary weapons activated (two Wizardspike).

I don't understand how this interaction works.

Even though he has his primary weapons activated, do the 2 secondary weapons apply MF?

If you can help me, I'd be grateful.

His MF barb plays "support" to his Merc. His main is two Wizardspike for the 100FCR and 150 all res. This allows him to Teleport insanely fast - remember the core of MFing is kills/runs per hour - and then tank in a pact of monsters while casting different warcrys to Weaken the monsters. This allows the merc to kill very fast. He regularly swaps to the two 6 Ist crystal swords and "horks" the corpses ( Find Item skill). Once the corpses have been used up, he moves on. If you look closely you'll see he is constantly swapping back and forth between the wizardspikes and crystal swords.

To answer your other question - no. You do not get the perks of whatever weapon you have a swap, unless you actually swap to them.

BTW I greatly enjoy this content creator. His video edits and humor are great

Thanks mhlg and Knappogue for answering my question.
I finally understood how it works.
You guys are awesome.