here's how to calculate, mathematically that is:-

you tint 65% heat rejection first and then on top of it another 82% tint. this means that sunlight passes through the 82% tint then only 65% tint.
light = 100% = 1
tint 1 = 82% = 0.82
tint 2 = 65% = 0.65

total heat after passing through 82% tint = 1 - 1x(0.82)
= 0.18

total heat after passing through 65% tint = 0.18 - 0.18x(0.65)
= 0.063

so in percentage, total heat that will go through both tints = 6.3%

if you tint 82% first and then on 65%, you'll get the same results.

but here's the catch, heat rejection rating is not equal to the visible light rating.....

you need to know what is the visible light rating/percentage for the tints because this is what JPJ uses to determine if you break the law. the law says visible light penetration through windscreen needs to be at least 75% if I recall correctly. if you know what is the visible light rating, you can use the same formula above to calculate the total visible light penetration.

but the problem is, car windscreen has its own filter value that sometimes causes not 100% visible light to go through even without tinting.

you will need to take this into account.

the best way to measure is to use the tool JPJ uses to measure the visible light penetration for your windsreen.

eg. if your tinting has 90% light penetration for 1st tint and another 90% light penetration for 2nd tint, and another 95% penetration for your own original windscreen, the total light penetration would be 76.95%.

hope this helps..........