AST doesn't need more of anything. What it does need is an identity of its own. It basically stole WHMs identity and added a card gimmick to it. Both of those jobs as far as their heals go basically play identically to each other and Yoshi has came out and said that healing has been a problem since AST's introduction into the game.
Healers in general need their identity back, because all they are right now is a green DPS. The focus is to heal as little as possible so you can focus on adding damage. While this is not something I have a problem with per se, my issue is the amount of downtime they have that allows for their DPS time to be around 90% of the encounter. Game mechanics focus too much on you fail you die, or become so weak as we gear up that they can be outright ignored. There is no middle ground and it is a huge problem.
It has been mentioned that healing basically comes in two forms: barriers and direct heals. You can toss HoTs and ogcds in there, but these healing methods are used to sustain DPS uptime, so I personally do not include them. Barriers and Direct heals are on the GCD. This is where I get stumped because I don't really know another way a healer that can use GCDs that are not barrier or direct heals. It can't really be HoTs because this type of GCD won't save a player who is at critical damage if an unavoidable comes raining down that finishes them off. Timed/explosive heals won't work because they would just be used to upkeep high DPS output much like Excog does now, and Excog is ogcd anyway.
What I have thought of is instead of having the MH/OH like you suggested similar to how tanks have a MT/OT, we have a Direct Heals (DH), and Barrier Healer (BH) dynamic. With this in mind AST would have to lose one of its secs, or its secs would have to work differently. A suggestion I have seen tossed around is that the two secs affect what AST cards do. We would then need a fourth healer to balance out the ratio of 2:1 DH or BH type healers the game would have at that point.
Right now, having two DHs isn't that big of a deal. Having two BHs though is a problem because shields do not stack. Barriers would need to be nerfed a bit so that they can stack with each other, and I feel shields providing more than 100% of HP restored to the barrier to be OP and just ridiculous. This can instead be achieved by another BH adding HP to the shield with their own GCDs. DH/BH would still be the ideal set up, but two BHs or two DHs can work because of the diverse utility they bring.
This then leads to healer utility, which needs to be spread out not only in order to establish diversity amongst the healers, but also grant balance through strengths and weaknesses amongst them that would allow the DH/BH dynamic to work.