You could dedicate channels to become parellel compression channels. For example, for a vocal you could dedicate two adjacent channels, assign both to the same input source (ie. the vocal mic) using digital patching, then apply the compression you want on the second channel. You could also stereo link the faders of the channels so that their level change proportionally to each other.
A similar thing could also be done with subgroups. You could assign the relevant channels to two separate subgroups and apply the compression to one.
