Here it's insurance cost that makes a second vehicle more expensive. I drive my truck that gets single digits for MPG every day, simply because insurance costs for liability only on a second vehicle is more than I'd save in fuel, or close enough that any maintenance or repairs would push it over to be more expensive. Not worth the extra time and headache if I'm not saving money.
Registration (road tax) doesn't make sense per vehicle either. It's a road tax for use of the roads, and here it used to be based on vehicle weight. A motorcycle was under $20, a car was $40-$60 and a truck was $60+ depending on gross weight rating. Now everything costs about $75, per vehicle. If I have tow vehicles I'm not using the roads any more, or putting any more wear on them... In fact I would wear the roads less since I'd be driving a 2600 lbs car most of the time instead of a 6000 lbs truck all the time. Far be it for the state to do anything logical, especially if that may reduce tax income.
I don't get it, because I can only drive one at a time, so insurance should not cost any more. Liability should be solely for the driver, in any car. Comprehensive/collision/agreed value/etc. should be per vehicle or based on the most expensive/risky vehicle you drive and cover all lesser vehicles. That would make sense though, and could hurt the crooks in the insurance scheme. One person can only drive one vehicle at a given time.