Got it! 😀
This passage is really really tricky. The key information is in the 3rd paragraph.
First, we want to add a compound that will INHIBIT the degradation of IKB -- in other words, a compound that will prepare cells for chemotherapy. Compound 1 does this.
Then, we want to add a chemotherapeutic agent: Compound 2.
So, add Compound 1 THEN add Compound 2. Prep for chemotherapy, wait a bit, THEN add chemotherapy.
To ensure that the cells are all first exposed ONLY to Compound 1, and therefore "prepped for chemotherapy", we should only expose them to Compound 1, and then wait a bit.
It's like swabbing an arm with alcohol... waiting a few seconds... and then taking a blood sample. You don't take the blood sample AND swab the arm at the same time.