Just tested and it is a bit long winded but it definitely works.
1. Assign key to switch player.
Install Keymap editor. PROGRAMS - Program add-ons - Get more... - Keymap Editor.
Run Keymap editor. PROGRAMS - Program add-ons - Keymap Editor.
Find Switch Player entry. Edit - Videos - Playback - Switch Player (bottom entry).
Assign key to Switch Player. Edit key - 0.
Save and exit. Back - Back - Back - Save.
2. You also need to have a suitable subtitle download add-on installed. This must write .srt files to the movie directory, not all do and if a .srt file isn;t written then the HiMedia wrapper won't recognise that there is a subtitle present. So..
Install Subscene subtitle add-on. SYSTEM - Add-ons - Install from repository - Kodi Add-on repository - Subtitles - Subscene.com - Install.
3. Play video.
Play any movie using DVDplayer. Find movie, press menu button on remote when it's selected and choose 'Play using' - DVDplayer.
Press OK on remote to get Kodi playback menu. Navigate to subtitle icon (bottom right, 4th icon from right). Select 'Download'.
Choose suitable subtitle from list.
Back out so movie is playing full screen again. Press button you mapped for switch player (e.g. 0 as above). Switch to HiMediaplayer.
When movie is playing in Himediaplayer press menu button on remote - Subtitle settings - Sub switching - Select subtitle.