Some processors support a sort of "hybrid" mode, where the capacity that's common works in dual channel mode (so you have 4+4 or 8 GB in dual channel mode) and the rest (12 GB) works as if it's in single channel mode.
When reading or writing to that portion above 8 GB, you'd get slower speeds but it will function.
Old systems (think dual channel with DDR2 times) had problems with dual channel with different capacities. They added support for these "hybrid" dual channel modes quite a long time ago. It's a bit fuzzy but I remember reading about such features in G41 (socket 775) chipsets from Intel and AMD had similar features around that time as well.