In TurboCAD V21, save a copy
of the drawing to the "DWG" format. Before you save, click on the "Set Up" button (Alt+U) in the Save As dialog and choose an AutoCAD format that your DoubleCAD program can read.
You could also try saving a copy of the drawing to "TurboCAD 18 for Windows…" in the "Save as type" field. It may work in an old DoubleCAD version, but doubtful.
You could upgrade to what DoubleCAD has morphed into, TurboCAD LTE
. LTE V7 and TurboCAD V21 can exchange TCW files.