Yup, 150kms more or less. No shuttle or bus service so you need a hire car to go between them.
I'd do two/three days in Coral Bay and then four/five days in Exmouth IF Exmouth Diving Centre is diving Lighthouse Bay and the Muirons. Unfortunately, they closed down again this summer and will reopen in March, but I do not know dates right now. My must dives for Coral Bay, in addition to the mantas would be Asho's and the Elbow, but be aware the Elbow is swell dependent so it's not always possible. In Exmouth, I'd do two days of "local" reef diving to Lighthouse Bay and then one day to the Muiron Islands and one/two dives on the Pier. If you're lucky, you can do the Pier in the afternoon after the morning reef dives. If you're really lucky, the Pier will be a day and a night double dive. It's tide dependent and I don't have the dive times for March but contact them and they should be able to help you.