jnfs1121

I like to do a 15/20 on the rear windows to help block headlights. Up til now, I only did 3M Crystalline, which comes in a 20% but is very brown, so I would do 40% for the 1st layer and 50% for the 2nd layer. No issues with defrosters. We just don't want to risk damage while removing tint from Tesla Model 3s. So if a customer comes back wanting darker, we add a layer instead of replace.

We only double layer on our personal vehicles...really only offered it once because a customer just tinted his car with us, but wanted it darker.  It was a brand new vehicle and we didn't want the risk of damaging the defrosters.  Other than that, they have to pay to have it removed if they want us to retint it.

What's with the Xpel hate? I've seen you pop up in multiple threads now saying not to use it but not offering any explanation for why?

As stated, double layering is typically not advised. However, many tinters do it, including us in select situations. Since the 2nd layer will only last as long as the 1st, we would only double-layer our own film. We've had Tesla Model 3s that wanted to go darker, so we added a layer to the rear window instead of removing the film risking damage to the defrosters. I do it on most of my vehicles since I prefer 3M Crystalline but don't like the heavy brown of the 20%. So instead, I add a double-layer 40 and 50 to get the darkness I want. We've never had an issue of peeling or otherwise. As far as absorption, I would not worry about it. FormulaOne Stratos, for example, has two layers of Ceramic in it, resulting in a higher absorption than the rest of the lineup but no complaints.

Double layering is not advised, however, it has been done extensively over the last 40 some years. The only issue I've ever experienced with double layers is when it comes time to replace it, the first layer will remove the scratch coat from the base layer. If you deal with a scratch coat that is known to easily scratch during installation, you might want to avoid double layering.
 
No matter what tint is on the glass; single layer, double or triple, the car's interior is going to heat up. Slower depending on the film's performance, but the interior will still achieve the 'hot box take your breath' away temps over sun exposed time.
 
Absorption level of film is a non-issue with tempered glass found in cars. The idea of absorption and how the performance of film plays out in the case of a car, is the faster the car moves, the more air passes over the glass, carrying away what has been absorbed. Hence, the car is cooler while in motion, but park that puppy in the sun all day and it'll still be hot when you climb in.
 
Remember; heat always seeks out cooler air until a balance is struck on both sides of any barrier.

It is all cheaper quality film but UltraVision would be the best option with a Lifetime Warranty. If you ever need to replace it, the warranty does not cover labor. So you would be paying for the removal plus installation for the 2nd time. Most tint shops won't install film they do not carry so I would call around to ensure you have somewhere to take it. Lastly, UltraVision is a thicker film which takes longer to prep so if the shop messes up during installation due to inexperience, you would have to buy extra film. Not trying to talk you out of it, just want you know it is not as simple as buying it and having someone stick it.

It's hard to say, you'd need to check with the shop. They may have cut you a deal since you were doing both at the same time, or they may have made a mistake and undercharged you. Regardless I wouldn't lose sleep over it. As far as the strip being done out of ceramic or non-ceramic again I wouldn't worry too much about it as it won't make a noticeable difference with the full windshield having been done in ceramic. 

I like visor strips myself, I'd go 5% film down to the AS1 mark, usually about 5 inches from the top.
 
As far as the rest of the car, I'd keep it the legal limit or lighter, but that's just me.
 
Reasons to keep your tint job within legal limits.