Whilst a route such as Ardrossan/Troon to Brodick will likely to be profitable enough to support one operator all year around, I would be very surprised if it would be profitable enough to support two. I doubt the routes from Lochranza will be profitable.
I think the best solution for Ardrossan-Brodick would be to have a lifeline contract which would be for locals only, with excess capacity sold only on the day before sailing to the general public. The lifeline contract/timetable/capacity would be set for a period of 10 years, and it would essentially cover the morning and afternoons, with the midday sailings being freight only.
A private operator would then be able to step in and run commercial sailings with the guarantee that the state won't interfere in their investment. Essentially, you'd allow a private operator to take advantage of the demand, but you'd still ensure that the islanders had a service with capacity for them.
Lochranza will never be profitable, but I doubt this would heavily interfere with the Brodick routes anyway.
Going back in history, but competition in the early 1980s on the ferries to the Isle of Man resulted in both companies (Manx Line and the Steam Packet) pretty much going bust. Enough profit for one operator, but not enough for two. They only survived by merging.
Yes, this is why I think there needs to be a state-owned lifeline operator to ensure that the island always has service regardless of what happens with a commercial operator. We know Arran has the demand for two large ships year-round, so if a commercial operator takes over midday sailings and a second ship's capacity, perhaps the tendered service could then be eliminated and replaced with a fully commercial service.
But even as I've just typed that out, I have the feeling that the easiest solution is simply to leave it alone.