The subject in this sentence, "two sources of stress," is third person plural. Verbs are only conjugated with -s for third person singular in English.

One source of stress applies

Two sources of stress apply

The subject is "sources", which is plural. You can break it down like this:

  1. Two sources  of stress  that apply to you.
  2. Two sources (of stress) that apply to you.
  3. Two sources             that apply to you.
  4.     Sources             that apply to you.

If there was only one source of stress, it would indeed be "A source of stress that (probably) applies to you". But because it's two sources, apply is correct.