From what I can see the program will only consider the first disableonsel statement. The rest are ignored.
This is what I came up with:
@disablenosel:numfiles=2,type=*.(3gp|3g2|asf|asx|avi|f4p|f4v|flv|m1v|m2ts|m2v|m4v|mkv|mov|mp2|mp2v|mp4|mpe|mpeg|mpg|mpv|mpv2|mts|ogv|qt|ts|vob|webm|wm|wmv|srt)
The above code will only enable the button when a video is selected or when a srt file is selected.
Given how common videos are though, it may be wise to just drop the requirement for having a video file selected altogether and just require having an srt file selected.
@disablenosel:numfiles=2,type=*.srt
I'm not aware of any way to have it so that it only enables when once of each is selected aside from making a script.
Alternatively, if you have a label set for a video or you add something to the name like [tosub], it's possible to use @disableif to disable the button unless either the name contains tosub or it contain the label you specify.