the average value should be reasonable

It is the 10 to 20 percent which would represent the % at 15 cm.

Sorry, it is the average of 0-10 and 10-20. Read your question wrong. Too early in the morning to answer questions!

The average of the 0-10 and 10-20 cm should provide a good estimate. Just be careful and realize that this is just an estimate as many factors (genesis, climate, etc.) can affect its real value.

I would recommend reporting both depths separately. Probably a big difference between the two depths.

You can also process as this: (a%x10 + b%x10)/20

The gravimetric answer depends on the bulk densities of the two layers