Their Body: "A dvati character is actually two separate dvati twins who share a soul. These two creatures move and act separately..." It goes on to say that they are exactly the same character class with the exact same build but their hit die is split between them when it comes to HP which means all other bonuses are added in to each twin. It also states they are a single PC so they have to spend double the gold to be equipped and they get only a single share of gold as well as XP since they are one character but two bodies. Because they are two different bodies, they get individual attacks and moves just like two different people. Because of this, I personally would give them a -2 to their Constitution because overall, they'd have more HP combined than a non-dvati would have. Example: A 1st-LvL Dvati Fighter with a 13 Con would have 6 HP (12 combined) and a Human Fighter would only have 11 HP. The higher the Con, the bigger the difference. Then you add in the possibility of toughness and psionic body feats and then class level if you go with your HP instead of skill points every level.
Now, because of this -2 to Con, they'd get a different ability score package. I'd suggest standard because "While dvati enjoy intellectual pursuits, they are capable warriors and hard workers. They consider archery, fencing, and similar combat talents art forms worthy of study." That means they should get a +2 Dexterity and +2 Intelligence.
On top of that, their Personality entry says: Dvati are typically serene, intellectually energetic, and curious. They tend to avoid conflict and are well suited to a role of peacemaker or diplomat. When faced with two sides in conflict, a dvati splits his twins. Each twin speaks from one side, finds areas of agreement with the other, smoothes over disagreements, and brings the two sides together..." I'd suggest giving them either skill focus (Diplomacy) as a racial feat or just a racial +2 to their diplomacy. However, giving them Diplomacy as a racial/class skill isn't without reason because of this.
My personal preference would just be giving them Skill Focus to the diplomacy skill. This is because of their outlook on things as said in the second paragraph of their personality entry "Dvati have a strong sense of duality in all things. They excel at seeing both sides of an argument. When faced with a problem, a dvati is likely to spend time considering multiple approaches from different angles. A dvati who must move a boulder that blocks a cave stands in front of it and surveys the scene. Meanwhile, the other twin climbs above the stone, looks at the stone from above, and considers the same problem. To dvati, one perspective is never enough." Which, personally, I'd give them the ability to reroll failed skill checks once as long as the skill check can have a second perspective to it such as an escape artist check if they are individually tied up with rope.
Their Mind: Right from the Dragon Compendium about their Psychic Link "The dvati twins have an innate, powerful psychic connection. They can communicate via telepathy at an unlimited range and across the planes. A twin cannot perceive what his other sees, hears, or otherwise perceives. As a full-round action, a dvati twin can determine his twin's current hit points and mental state. If one twin is on the same plane as the other, the twin learns the relative direction and distance to his partner." This means there is no limit to their mental connection when it comes to communication. This means, with a proper communication to the party, the local planar expert can determine which plane the other dvati twin is on.
With magic items, sensibly, they each individually are affected by whatever they are wearing. That means if one is wearing a magic item that gives them a +2 Charisma, only that one is affected.
With armor, if they are a spellcasting class and one is wearing heavy and the other isn't, the one wearing the heavy armor would get the arcane spell failure chance, not both.
Governing their favored class as a bard in 3.5, I'd suggest giving them a +1/6 uses of Bardic Performance for dvati bards.
Possible Racial Feats
-Some sort of ability that allows half the dvati to cast a spell.
--Which would be the prerequisite of allowing half a dvati to maintain control of the spell or concentration effects as actions would disrupt the concentration.
-Possible ability to aid their twin with skill actions as long as they have skill focus with the skill or have to take this feat for each individual skill and must have skill focus in that skill before taking this feat for that skill.
-If you keep the Echo Attack, they could have a feat that turns the benefit into 1d6-1 sonic damage that bypasses hardness and in turn causes them to be exhausted when used in this way but can only be used once per day per number of times this feat is taken.
--Which could lead to them being able to take a feat that makes them fatigued but they'd have to be level 5 and requires the sonic damage feat to be taken twice.
---Then they could take a feat that causes them to be unaffected by the echo attack when using it as a sonic attack but requires them to be level 11 and have the sonic damage feat taken 4 times.
I think I've caught all that's needed.
Note: I made this comment while using the "Dvati" entry in the "Dragon Compendium."