First this is not my glitch, I just searched youtube and found a video tutorial of this glitch. The video is a little long so I'll write the steps out here and post a link to the video at the end. Even the creator of this video says that he wasn't the first to discover this.
Second and most important for this glitch if you have already started Veronica's companion quest or Veronica is dead in your game then this glitch cannot be used and you'll need to reload a previous save or start a new game.
Step 1: Head to the 188 Trading post and recruit Veronica. You'll then need to trigger 3 events so that she approaches you about her companion quest.
Step 2: When she asks something along the lines of "if you have a minute there's something I want to talk to you about" say not now.
Step 3: Fire Veronica and go hire another humanoid companion.
Step 4: Go back to the 188 trading post and talk to Veronica. Select the option "what did you want to talk about earlier?", she'll say she wants you to go to the BoS with her. Select the option that she should go alone, she'll try and convince you again and select you won't go and she should go alone. When this happens it will say you've lost Veronica's Scribe Assistant perk even thought you didn't have her as a companion, this seems to create another slot for a companion.
Step 5: Talk to Veronica again and select "I think we should travel together" and you'll have 2 humanoid companions. continue to repeat steps 3-5 until you have all the followers you want.