Is it a bad idea to take one of multiple plants outside for a day or two and reintroduce it to the indoor tent? Could it cause damage to the other plants?
Its a bit odd only for a couple of days; moving to the outside usually requires a couple of days of hardening off alone. What's the reason for taking one plant outside just for a couple of days?
it is certainly possible, depending on the particularities of the specific outside to which you are taking her, to 'catch' some pests or diseases that she could then bring back to your indoor space.
I do it all the time in veg weather permitting. But I'm spraying weekly with neem so that helps prevent pests. I don't do it once the plants are flowering.