If I understood what you want to do (2 subs showing at the same time), you can click for each subtitle copy and paste. This way, another copy of the same subtitle is created. Just drag it with your mouse for another corner of the screen and type whatever you want for the second language (if that's the case). This process lets both subtitles show at the same time, but as far as I know, you will not be able to choose among one or the other one since both will show at the same time. And there's one more thing: there's just one indication for language for each stream.
;)