As @psb6m suggested, ligatures are typically multiple glyphs that get substituted by a single glyph. This is a many-to-one substitution.
OpenType does not offer a many-to-many substitution as you want, but it can be simulated by placing a lookup in the Prefix section and referencing that from the ligature feature:
Here, you match the pattern A G in the liga feature. The ' single quote marks the G as the glyph that should get replaced and the lookup some_ lookup after the pattern references a lookup that you have defined in the prefix which substitutes the G by all of the desired glyphs, including the G itself.