Identify the selected engine
SMPlayer is a graphical frontend that delegates playback to an underlying engine. Its current support page describes selecting mpv in the General area of Preferences when you want to use it instead of MPlayer. The name SMPlayer alone does not identify the active backend.
Record the frontend and engine versions before changing that choice. The publisher’s original dual-subtitle announcement explicitly requires mpv, so a feature comparison needs the engine context. A screenshot from another installation may show controls unavailable in the configuration you are currently using.
Check that two subtitle sources exist
Identify the subtitle tracks actually supplied with the authorised video or external files you are using. An engine capable of two subtitle streams cannot create a translation that is absent from the source. Keep the language names and the chosen first and second tracks in your notes.
The publisher’s dual-subtitle description places one subtitle at the bottom and the other at the top. Use an ordinary short sample to check whether the selected tracks appear as expected after confirming the engine. If they overlap or remain absent, record that specific result rather than assuming the whole media file is corrupt.
Keep a version-aware comparison
The original announcement dates from 2014, while the current engine-support page still lists the feature. Its historical introduction instructions should not be substituted for the current Preferences route. Use the controls exposed by the version you have installed.
If asking the project for help, provide frontend version, selected engine, track type and what each selected subtitle actually does. Preserve the original settings so a comparison can be reversed. This separates a backend prerequisite, an unavailable translation and a rendering problem, rather than repeating a general reinstall whenever a second subtitle does not appear.