OK, you have 2 charts(one for each driver). Whoever is actually driving is No1. The 'passenger' is No2. If you are both on rest it doesn't matter which is which.
When you open the tacho, there is a thick plastic flap (that your disc fits on if you are a single driver). Flip this open and insert the disc for the non driver. Close the flap and insert the disc for the driver. The knob for the driver operates as normal, the knob for driver 2 has no 'drive' function. Personally, when my wife and I were double manning I always left this on rest. I tried to get confirmation I was doing the right thing from VOSA, but they said it was a 'grey area' and I never had any problems with the law (UK or Continental). So, leave 2 on rest and use 1 as normal. Whenever you change drivers, you MUST change the discs over. Your main advantage is that you can both do a 9hr (or extend to 10hr as usual) day, and No2 is having a break while 1 is driving, but you MUST BOTH have at least an 8hr break together in any 30hr period . For more info, go here and look at items 16 & 20.
http://www.dft.gov.uk/stellent/groups/dft_frei ght/documents/page/dft_freight_504543.pdf