Aside from volume, the Exodus is a an extremely similar pack to the Prophet you already own. According to MLD's website, the Exodus is only 0.5oz heavier (maybe 2-3oz heavier with your mods?). If you really need the extra volume for some trips, maybe sell your Prophet and just get the Exodus instead of owning both. If you own both, you are essentially keeping the $180 Prophet just to save 0.5-3oz on some of your shorter trips. Perhaps these funds could be better utilized?
If you do want a two pack quiver, I would encourage you to get ones that differ more so that you can cover a wider range of trips. If you keep the Prophet, I would add something significantly more substantial (ie. ULA Circuit) which would be ideal for even longer trips and/or winter trips. Frame stays aren't mandatory at 25 lbs, but they are quite nice to have. The Exodus just doesn't seem different enough to warrant keeping both the Exodus and Prophet.
I think the ideal two pack quiver includes a small, frameless pack under 10oz (ie. MLD Burn, Zpacks Blast/Zero etc) for those 1-2 night SUL/UL trips, and then a larger pack with frame stays that will accommodate anything from 3-10 days. Something like the ULA Circuit, HMG Windrider etc. You might want to sell the Prophet and buy both smaller and larger packs.
I personally use a 4.5oz Zpacks Zero (with options like water bottle pockets) for 1-2 night trips, and then my ULA Ohm covers everything from 3 nights up to ~10. If I'm up around 7-10 nights then I do wish I had something even more substantial, but these trips are infrequent and the mild discomfort is only for the few couple days.