Using Muscle Milk as meal replacement in the morning and Syntha-6 as post-workout supplement at night?

I've been using syntha-6 for at least a year, love the taste, even with just water (full bottle of water + 2-3 scoops). Parents surprised me with a 4 pounder of muscle milk (costco had a coupon for it) and I've been researching it a lot and don't know what to think. Maybe I should return it. Then I had an idea. What would happen if I drank muscle milk as a meal replacement in the morning, then drank syntha-6 after workouts as I usually do?

On the topic of meal replacement, am I supposed to still drink it on rest days?