DRUMs are ALWAYS controlling their outputs. So when it is "disabled", all of the DRUM's outputs are turned OFF.
Hence, if you have 2 drums controlling the same output, the last one wins.
If you stuck them in 2 different stages that are mutually exclusive (neither or just one of the SG are enabled, but never both), then it could work.
An alternative is to use the OUTPUT MASK which is a set of bits at each step for determining IF the DRUM CONTROLS EACH INDIVIDUAL OUTPUT. This is a way to "override" the DRUM having control (e.g. for manual override, which is sort of what you are wanting to do). Basically, you can tweak a bit for a specific output at a specific step whether the drum's ON/OFF is written to the OUTPUT bit (otherwise it does not write ANYTHING to the output, neither an ON nor an OFF). Let us know if this is something you might consider. I do not know if this option is better, but chances are the 2 different stage approach may be easier.