You will need a way of connecting both your current 2TB disk and a new one at the same time. A USB hub (if you don't have free USB ports) and second enclosure, or 2-bay disk dock (much cheaper than a NAS device and no networking required) will do.
You can then combine their storage with mergerfs (available for most distros). Both disks will still work independently, and you can use indexing software like gwhere, cdcat or gcstar to scan each drive so you can tell where a particular file ends up.
You might also be able to buy yourself some more space by using jdupes or rdfind to hardlink duplicate files.
It's a clbuttic