Okay so over the last few months I've been trying out heaps of different bags to see what suits me and I think I've finally got er done!
After trying a few different bags (Crumpler 6MDH, Thinktank Retro 10) I eventually went back to the drawing board and settled on a Timbuk2 Snoop Insert for my Crumpler Considerable Embarrassment messenger bag. I already had the bag and use it to carry all my stuff to work so it seemed like it would be a good idea to put it to use for my camera equipment.
The insert arrived today from Amazon and I'm very happy with it, I can easily fit 2 cameras in with about half the space left over for additional stuff like film, meter, filters etc (see pic below). Comes with 2 large and 2 small dividers for laying out the compartments so it's very easy to configure it to suit your needs. The thing I like most about this setup is that it really doesn't look too much like a camera bag, it moulds nicely against my body so it just looks like any other messenger bag. Fits PERFECTLY in the Considerable Embarrassment too, the small size slides in perfectly with no wasted space inside the bag.
For $40USD I think these things are a steal, when I don't want to take my cameras with me I can just slide the insert out and use it as a day bag so that I don't have to unpack all my kit everytime. Will be good for travel too, I still have plenty of space left over for battery chargers and a laptop if I'm going to be away for extended period of time.