The Box command only groups in one category at a time, so you will need to have two Box commands, where each one has a mask and is offset to overlay the bars.
To do this, select the box command you already created with the points option. Expand the command and add a negative offset and a mask to limit to the appropriate data.
Then you can make copy of the Box command (select and use command-C and command-V). Edit the copy to have a the same offset but positive. And update the mask.
Does that help?