Just came across this old question and thought I’d add my solution, which may be useful for somebody in the future.
Sublime supports searching in all open folders and can use regex. So utilizing both, you can open or add all folders you want to search in to the project and use regex to search for multiple keywords. In your case it would be the following (make sure to check the regex box .* icon):
Find: (string1|string2)
Where: <open folders>