Actually, Super User has a multiple monitors tag: [multiple-monitors] (435 questions). This is the tag to use for discussing setups where multiple monitors are being used simultaneously. We use the [monitoring] tag for questions about monitoring software, but they can be easily confused so it's good to watch out for that.
Several of the existing [monitors] questions already have the [multiple-monitors] tag as well, and a couple of the others should have the multiple instead of the singular. I've fixed those manually, and batch-retagged the [monitors] tag into [monitor].
Thanks for finding this.