Twin dragons Valiona and Theralion is rather an easy Cata encounter. Good for those who wants to try out Cata bosses. There isn't much to deal with in terms of mechanics. There is always one dragon on the ground and one in the air. The primary attack target will be the dragon on the ground but it is highly encouraged to dot the one in the air not only for that extra DPS, but also for extra mana from VT and extra Mind Blast procs from SW:P (or extra Mind Spike procs if FDCL is talented).
When Valiona is on the ground:
The only major issue is Blackout debuff. The debuff lasts 12 seconds but it can be dispelled or removed by absorbing 75k healing. If this debuff is removed in any way it will deal 200k damage, split between between the raid. But since there is nobody else here to soak with you, well...you gotta take it all by yourself. This damage is not really dangerous when you have powerful CDs such as Desperate Prayer, Dispersion or Vampiric Embrace.
When Theralion is on the ground:
Nothing really dangerous here. Theralion will cast a debuff called Engulfing Magic. This debuff will allow you to do double damage and healing, but the consequence is that all raid members nearby will suffer the same damage that you deal to the boss. This means that not moving out of the raid when having this debuff will cause a quick wipe. However since there is no other raid members to worry about when soloing, this debuff (or buff rather) is essentially a free +100% increase to damage and healing. With that kind of buff there is no way you can die.