For a while, manjaro32 has been combining archlinux32's stable and testing branches when syncing into x32-unstable.
For manjaro32, this has benefits of getting updates through quicker as archlinux32 build from Arch stable as well as highlighting issues for upstream to address (e.g. fixing build issues).
There's a possibility to do something similar with Manjaro: combine Arch stable and testing into a new "fragile" (tbc) branch.
This would bring in a number of potential benefits, principally spotting packages with issues which can then be replicated, tested, and fixed upstream. It should also tie in to Arch [testing] team so you don't need necessarily need to be running two separate installations.
However, unlike manjaro32 where packages are already relatively stable, doing this in Manjaro would likely end up with regular breakage so would probably only be for the most enthusiastic/brave/foolhardy/extreme.
Essentially, it would be like running Arch testing, or Debian sid+experimental. So, it can be done.
Potential issues with building packages should be mostly addressed by building against unstable then pushing to unstable and fragile at the same time; it should only really create an issue for fragile when there are large toolchain changes.