The feature file spec (5.f.i) says:

For each glyph or glyph class in the input sequence, the contextual rule may specify one lookup (§4.e) to be applied at that position.

However, the OpenType spec allows more than one lookup per position. ("multiple substitutions may occur in a given context", Substitution Lookup Record)

Syntactically it should not be a problem to extend the feature file spec to multiple lookups and allow e.g.

sub a' b lookup l1 lookup l2;

(unless someone has a glyph called lookup in which case they get what they deserve).