You might want to check that you have the following setting set to false
in your user settings:
// OS X only: When files are opened from finder, or by dragging onto the
// dock icon, this controls if a new window is created or not.
"open_files_in_new_window": true,
As seen here, it defaults to being turned on in accordance with how applications generally behave on OSX. You can use Sublime Text > Preferences > Settings
and check that it’s in your user settings in the right hand pane, and set to false
.
That said, although I don’t have this option disabled on my Macbook (I never open files this way), dragging multiple files to the dock icon in one drag operation opens a new window and puts both of the files into it. If you’re seeing that operation open two separate windows, this setting may not help.
I’m running the latest development build 3154 on MacOS 10.10, so if you have a more recent version of MacOS (and perhaps updated it recently) there might be something MacOS specific that’s caused the behaviour to change.