Hi Nick,
Yes, you can do this. Go to 'Edit | Edit Data | Phase...' and create two phases* - use the 'Unit cell' and 'Structure parameters' tabs to create the two structures (in this case, Si and GaP) as normal. At this point, you will have one unit cell of each, sharing a common origin. You can reposition and reorient the phases using the options on the 'Phase' tab and change the number of cells displayed by using the 'Boundary...' dialogue and choosing the relevant phase from the drop down list at the top.
Hope this helps,
Philip
* For some reason, renaming the phases by typing in the top box doesn't work - perhaps this is something Koichi can look into? To rename a phase, double click on its 'Title' field until it becomes editable.