Originally Posted by echang
You wouldn't necessarily have to rely on choosing a banner. For example, they could design it so there are two sections on a banner pole, where the top is your personal army banner and the bottom is the guild banner, or vice versa.
This is pretty much how I envisioned it per the main suggestion. That would be really cool to have each column of your army have one main banner and under it a slightly shorter banner that declared what guild you were in.