Connecting two analog multiplexers (CD4051b)
Each CD4052B multiplexes 8 input to one output, and you need three digial pins to control it.  You have the six analogue input pins available and those pins can also serve as digital outputs, so you should be able to use  them to do what you need.

I would do as follows:

Assign three of your spare pins to be digital outputs.  Use these three to address both CD4052Bs together -- so both devices will receive the same addressing.

Connect the output of one CD4052B to the fourth analogue pin, and the output of the second CD4052B to the fifth analogue pin.  These will be your inputs.

On both CD4052Bs, tie the INHIBIT pin low, so they are permanently on.  (If you want to be able to switch them off, for some reason, you an use your sixth pin to control the INHIBIT line on both devices.)

Now when you want to read from sensor 1, you set 000 on the address lines (A, B and C) and read from the first input.  At the same time you'll have sensor 9 on the second input.

It would be well worth reading the data sheet