Is it common to own two MTBs for different weather conditions? I'm newish to cycling, so I apologize if this is a stupid question... Does it make sense to have two MTBs -- one for dry weather and another for wet/winter weather? I'm doing both paved and dirt trails.

I'd like for my current MTB (a 27.5x2.0 hardtail Specialized) to be used in dry weather since it already has slicks on it. Then I'd possibly like to have a second MTB with much wider tires (e.g., Trek Roscoe 7 @ 27.5x2.80) for the rainy/wintry weather. I don't want a fat bike per se, but something with wider tires so I get more traction, and likely studded tires for the winter.

I'm not riding everyday because I'm afraid of slipping in the rain, and I'm kinda lazy about swapping tires. I know it only takes like 5-10 mins to swap tires, but I don't even know if it's safe to be swapping them all the time (e.g., if it destroys the rubber over time). Also, having an extra set of wheels can be expensive considering my current MTB is only $450.
This can not be answered objectively for all people. Your current conditions, budget, space availability and preferences all play a role in such decision.

The advantage of having just a spare wheel set is space saving. If they are for disk brakes then they are the fastest to swap.

The advantage of having more bikes is (more bikes! Duh!) you can have them tuned for different purposes or types of riding. Even the same model of bike you can tune more for aggressive climbing and other for comfort (long rides) or for aggressive downhill practice. Switching from one bike to another is less hassle than having to re-tune the bike every time you decide to go for a different type of riding.

Another advantage of multiple bikes is that one of them breaks in such a way it is difficult to repair, you can just ride the other one while the broken one is under service or waiting for spares to arrive.

Some people do have a bike for training and other for racing. I do not carry that practice, so I do not know the specifics, but I'd think that they have the top-level bike in a safe place while the other one takes the risk, wear and tear of routine training. The bikes would have very similar configurations but maybe the training one is heavier, lower spec'd components and maybe use on that one the tires that aren't new but still have some use life on them.