We associate in a canonical way a substitution to any abstract numeration
system built on a regular language. In relationship with the growth order of
the letters, we define the notion of two independent substitutions. Our main
result is the following. If a sequence x is generated by two independent
substitutions, at least one being of exponential growth, then the factors of
x appearing infinitely often in x appear with bounded gaps. As an
application, we derive an analogue of Cobham's theorem for two independent
substitutions (or abstract numeration systems) one with polynomial growth, the
other being exponential