Markerlights in fluff and rules are used to "light up" a target. This makes the networked gun systems of all the Tau be super accurate against that specific target. I don't have my book on hand, but the Markerlight section of your codex should go into detail on how this is used.
Therefore, the markerlights on Target A can only be spent against Target A and no other target - even if you use Target Lock to attack a different target.
If the question is then, Target A has markerlight tokens and Target B has markerlight tokens, may I use tokens from both units to increase my shooting via Target Lock split firing if I choose to shoot at Target B? I would say yes, but I would need to look at the codex entry to double check.
Edit: Pg 123. Yes, split firing (target lock) models may use markerlights on the target that they are shooting at. Think of markerlights as something that "debuffs" the target. It does not buff your unit itself.
Keep in mind that if you plan to use Target Lock shooting you must nominate all of your targets before you start rolling to hit.
Edit: As far as tactics go, using one unit to shoot at multiple units that have multiple markerlights on them might be a damage decrease, rather than an increase.
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