Due to how the level 1 thermometer works, I would prefer to bring one to help check for temperature, but I also really enjoy using the higher level thermometers to help figure out which room is the ghost room. Is there any way to mix and match my two thermometers and have them be different levels?
If you have multiple people, you can mix tiers. A single person can only bring one tier of each item, but each person can bring different tiers to each other.
You need multiple players to use multiple tier types; And that part is on purpose atm. The devs may change their mind in the future, but for the moment, if you're a single player, you get a single tier of each item.