So I've gone over this whole thread trying to figure out why this is so confusing. The designers couldn't possibly have intended for people to deal that much damage each turn. It's not like this is even some combo with an obscure feat that they didn't think about. This is the archetype as-is.
As it stands, I can't see any justification for why Thunder Call & Inspire Courage can be used in the same round. Thunder Call is a Bardic Performance. Inspire Courage is a Bardic Performance. The rules are super clear on this.
PRD wrote:
Each round, the bard can produce any one of the types of bardic performance that he has mastered, as indicated by his level.
PRD wrote:
A bard cannot have more than one bardic performance in effect at one time.
That's the RAW. If you want some kind of precedence to assess RAI for yourself just look at the Bardic Performances that have immediate effects.
PRD wrote:
Suggestion (Sp): A bard of 6th level or higher can use his performance to make a suggestion (as per the spell) to a creature he has already fascinated (see above). Using this ability does not disrupt the fascinate effect, but it does require a standard action to activate (in addition to the free action to continue the fascinate effect). A bard can use this ability more than once against an individual creature during an individual performance.
That's a long quote, but you'll notice that TWICE it makes an exemption for the fact that it doesn't stop the fascinate performance. Thunder Call does not mention the same exemption.
Clearly the intent was to give you the option to deal damage with your performances not deal damage AND keep buffing your allies.
One more example:
PRD wrote:
Inspire Greatness (Su): A bard of 9th level or higher can use his performance to inspire greatness in himself or a single willing ally within 30 feet, granting extra fighting capability. For every three levels the bard attains beyond 9th, he can target an additional ally while using this performance (up to a maximum of four targets at 18th level).
By the way everybody else is interpreting multiple performances of Thunder Call, I should be able to use Inspire Greatness on multiple allies by simply starting the performance multiple times. If that was the case, why does it start granting additional targets as you level? Wouldn't the additional rounds of Performance you get as you level scale it well enough? And even then, why would it put a cap on the number of targets at lvl18, when you're doing it to three people by lvl13 following the rules people are using for multiple Thunder Calls?
The truth is that Thundercaller is a side-grade from vanilla Bard that makes you give up suggestion so that you can contribute some damage when the party doesn't need you to be Inspiring Courage. You know, because you can't do two performances in one round.