Yes. You must be enrolled in and/or have completed both units within a 24-month span.
For example, if you start Unit 1 in January of 2019, you must be enrolled in (or have completed) Unit 2 by December of 2020. Extensions are available if life gets in the way.
But we make this rule for the sake of continuity between exam and manual. We update the manuals every two years. We need to ensure that you take the exams for the manuals you have studied.