The answer to your question about layering is yes, you can add them together.
Actually, if those 2 garmets were tested together as a multilayer garment they would probally have a rating around 40cal, but there is no way to know without actually testing them, but you can get your 23.6 cal.
I agree with Ron that you must be careful using the tables in the 70E, there are alot of limitations and you still need to know the fault current and clearing times.
There is a free calculator that uses the 70E equations at this link
http://www.epowerplus.com/Training.htm/download.shtml
