737NG - Dual Channel AutoPilot ?

Hi,

I currently can't access the on-line tutorials for this plane as, although I do have the Internet here in the office, I don't have WinZip to unzip it ! (Not allowed to download WinZip onto office PCs)...

Can someone tell me how to engage BOTH AutoPilots on the PMDG 737NG, therefore effecting a DUAL CHANNEL landing ?

Even on Approach mode, when I click the second AP on, the first one switches off !

Thanks for any help....