It doesn’t matter whether you are using American English or British English because you can use both focussing and focusing in either variant. However, it is much more common to find the single “s” spelling, focusing.
This is also true for the simple past tense and past participle of the verb “focus.” This can be spelled focussed or focused, with the single “s” version also by far the more common form.
The QuillBot Grammar Checker has selectable language options to make sure you have the correct US or UK spelling of focussed or focused.