Tactical Advantage

T.A. can be used on these abilities exactly:

laceration
carbine burst
stim boost
kolto infusion
surgical probe
cull

However the wording is a little confusing only to the effect of this:

Does the 2% damage boost stack with the stacks of T.A.?
If it does that could change the damage rotation around for skills such that you would get the optimal damage output for the first few abilities like this:

Hidden strike
Shiv (to apply the second 2% damage increase)
Backstab (using another 2% damage increase vs using it prior to Shiv)
Corrosion Dart (to reapply a poison to take advantage of Tactical Opportunity that people seem to forget exists with a need for a poison)
then the usual Laceration spam blah blah

basically it would give you additional damage on backstab but every little bit helps especially when you are hitting for 3k crits. 2% of 3k is an Additional 60 free damage from that one ability alone (not counting stacking cumulative damage modifiers to make it in excess of more) simply by changing around the rotation.

The question was "Does the damage modifier stack with the T.A. stacks or does it only apply the 2% once?"

The proper usage of tactical advantage can allow the skillful operative to strategically take out player or NPC opponents.

By default, you gain a T.A. from three sources as mentioned in the tooltip above (defeating an enemy, shiv, or hidden strike). Gaining specific talents can add opportunities to gain a T.A. as well. For example, putting talents into kolto injection reduces the activation time as well as allows it to grant 1 T.A. Additionally T.A.'s can come from surgical probe with the proper talents allocated. There are other talents that grant T.A.'s as well, look through the talent trees to see where they are and how you might want to use them!

The tactical advantage not only provides a nice to have 2-4% damage bonus while it lasts, but also the ability to enact skills that are otherwise unavailable, such as:
kolto infusion
laceration
carbine burst
stim boost

and others. these skills are versatile, and when used properly will allow the operative to handle numerous situations with ease and be one of the most valuable players.

with limited knowledge of the game inner mechanics, these two lines under the modifier application subeffect in the ability details make me think you can have up to 2 charges of TA, but the effect only stacks to 1

SetCharges: InitialCharges=>1, MaxCharges=>2
SetStackLimit: MaxStackCount=>1, IsByTag=>1, IsPerCaster=>1, RelevantTags=>tag.abl.agent.finishing_move

Also further ahead on action subeffect:

ModifyStat: AmountMin=>0.02, Stat=>Damage Done Modifier (Percentage), AmountMax=>0.02

that's your 2% increase, but again, limited knowledge, so maybe for the game that line sets the AmountMax=>0.02 per charge and not per whole buff...