I guess the occasional cup of juice is fine if you want to keep your energy up, but drinking juice, which is very high in sugar, prevents you from going into ketosis, which is what decreases your appetite and makes pure Water Fasting easier after the third day. So even though alternating might seem easier, I think it would ultimately be harder because your cravings would never stop and would make you miserable for all 35 days. I've tried juice fasting where I would only drink around 2 servings of fruit juice a day, and it was sooo much harder than Water Fasting alone. But it could work for you. Good luck!