Create (Start) a multisession sequence.
Then give it a folder name and click New.
In the panel the Folder name will be in the top left hand corner and your hard drive's contents listed on the right.
Drag and drop your original file into the folder and then add your other file. Both will be listed in the second column.
If the size of the files fills the disk choose Finalise from the Multisession tab; otherwise tick the Add Files box on the same tab.
You can now burn the disk(s) as required and, if you later want to add further files, choose Continue Multisession after opening the original Folder.
Don't forget to Finalise just before the last burning of files if the disk is about to reach full capacity.
Then burn the disk or disks.