"Plan B" in GlyphsData decomposition

I want to have more than one decomposition rule in GlyphsData.xml.

For example, quotedblbase has “quote, quote” as decomposition value, but I want to use “quotesinglbase, quotesinglbase” as primary rule and “comma, comma” as a secondary option.

I can just think of this one particular case for now, but how else would this be useful?

This specific use case is not strong enough for me to add that feature. If there are stronger examples I have a look.