A new documentation is available for Squash TM 2.0 and later versions, check it here
To enable the selection of more than one project when specifying the criteria for the generation of reports, edit the Squash TM configuration file (Squash-TM\conf\services\squash.tm.cfg.properties) and set the following option to 'true''
report.criteria.project.multiselect = true
(to only enable the selection of one project, set this parameter to 'false')