It depends on what you are doing with them. If you want to create one large desktop side by side and have a smooth visual transition from one display to the next, I would get the same res and same size. However, if you just want something to the side for extra stuff, it doesn't matter. I am assuming you are not gaming on both at the same time, because if so you will NEED them to be the same res.
The brands do not have to be the same, odds are their LCD panels game from the same place. I think 3 companies actually make LCD panels. I personally have a 22" and a little 15" in portrait mode as a spare and overflow that is usually off and disabled. However, when I need more room I turn it on and enable it. It has a different res so it is setup as an extended desktop, which means it has it's own resolution and runs as an independant monitor. If it were in the same group as an expanded desktop, the display would HAVE to be the same resolution and it would act as one single large desktop.