fontlab opentype contextual alternates

hello...

I'm attempting to make a font with some OpenType features for the first time, using Fontlab, and, guess what - it's slightly confusing...

what I'd like OpenType to do is:
for lots of specific character pairs, substitute pairs of alternate characters

so, for example, if someone types 'AB' I'd like OpenType to substitute glyph A_2 followed by glyph B_2...

I was kind of thinking I could do this with a contextual alternate feature like

sub A' B' by A_2 B_2;

but that doesn't seem to compile

sub A B' by B_2;

compiles OK, but doesn't really do what I want...

does what I'm trying to do make any sense? is it possible?

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