If you want to get a combined report, then please add the command line switch /GROUPSCANS true to the beginning of the command line.
If you want seperate reports for those drives, please split it up into separate TreeSize calls.
Unless instructed otherwise TreeSize deletes all files to the Windows using the same mechanism as the Windows Explorer. That means, if according to your settings files get deleted to the recycle bin, they can be recovered from there.
Since we do not know which files you have on your disk, and also do not know which files you or your colleagues or family members might be interested to keep, we cannot give you advice here.
We also do not know which files may be required by some program that are installed on your system.
The results of the TreeSize Temporary File Search and the Browser Cache Files are quite safe to delete. If you delete files, only delete them to the recycle bin or move them to a different path, from where you can recover them again a few days after you deleted them, just in case you deleted a file you still need. The "Move Files" feature of the TreeSize File Search and the TreeSize "Details" list makes this easy.
Let the first user export his history through "History > Manage history data > Export to file". The second user then can import this file through "History > Manage history data > Import from file", the data will get merged.