The Flex bag is quite big. I have one and putting eveything that you want inside would render that bag quite heavy. Do keep in mind this is not a bag with stiff walls like the autogeek bags. But they are great none the less. If you want to put both flex and udm side by side laying flat I would say that is not possible. Side by side laying on their side could be possible but it will definitely be tight. Especially if you start adding pads and polishes. I would recommend you also put some spongefoam or bubblewrap at the base of the bag as there isn't much padding there and it was just hurting my heart when I would heAr the flex hit the floor whenever I put it down with the bag.
So in response to your question, is it possible? Possibly. But in my mind that bag will be overloaded and flex designed this bag for convienient and safe travel for only one D.A.