As mentioned in the Strategy Session 1, one should first complete a specific section and only then move on to other sections.
You can watch the recording of Strategy Session 1 from here: GMAT Strategy Session 1.
This implies that you should NOT do SC and CR sections together. If you are preparing for SC, then focus only on SC. Likewise if you are preparing for CR, then focus only on CR.
For effective preparation, follow the guidelines provided in Strategy Session 1.