Add windows to multiple groups

Quoting the man page (0.9.23): "Note that launching applications from the startup file is acceptable, but can be of limited use if they have to start in more than one group" It seems that currently there is no way for a window to belong to more than one group, i.e. to appear in multiple specific groups. It would be lovely to have one window appear in two different "work spaces", to be able to collect information from or add information to certain other windows on a regular basis by simply opening a certain group and have the desired windows appear side by side reliably.