Contextual alternates - substituting both glyphs in a pair?

I’m trying to substitute both glyphs in a pair when using the calt feature, for example:

sub Q’ y’ by Q.alt y.alt;

It’s giving me a “many to many substitution not available” error. Does that mean this isn’t possible? I can create a ligature instead if I have to, but I’m hoping I can do it with calt.

You need to split the substitution into two substitutions:

sub Q' y by Q.alt;
sub Q.alt y' by y.alt; 
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