Is doubt about disbelief, and refraining from repeating the Two Testimonies due to satanic whispers, and uttering them only once while proceeding with the belief, considered a valid action?

It is clear that the questioner is afflicted with waswasa (whispers/obsessive thoughts), so she must ignore them and pay no attention to them, for indulging in them leads to great evil. If she doubts whether she committed an act that necessitates disbelief (kufr), the default is that she remains Muslim, and she must act upon this default and disregard the doubts. Whoever commits an act that necessitates disbelief and then pronounces the two testimonies of faith (Shahadatayn) has returned to Islam, regardless of her devotion (khushu').