You get to decide which damage to prevent. And you don't have to prevent all damage from one source before preventing damage from a different source.
The first thing to note is that creatures do not attack in order - it just seems that way when you assign damage to blockers. Attacking creatures are required to assign as much damage as is required to kill each blocker, but all the damage is dealt simultaneously.510.2 Furthermore, there is no assignment step for damage that will be dealt to a player.
The next thing to look at is the way that damage is actually processed. From the comprehensive rules:
119.4 Damage is processed in a three-part sequence.
- 119.4a First, damage is dealt, as modified by replacement and prevent effects that interact with damage.
- 119.4b Next, damage that's been dealt is processed into its results, as modified by replacement effects that interact with those results (such as life loss or counters.) [ed: These are detailed in 119.3]
- 119.4c Finally, the damage event occurs. [ed: examples follow]
The final piece of the puzzle is in the rules regarding Prevention Effects (emphasis mine):
615.7 Some prevention effects generated by the resolution of a spell or ability refer to a specific amount of damage—for example, “Prevent the next 3 damage that would be dealt to target creature or player this turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” creature or player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be dealt to the shielded creature or player by two or more applicable sources at the same time, the player or the controller of the creature chooses which damage the shield prevents. Once the shield has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount of damage; the number of events or sources dealing it doesn’t matter.
Putting it all together, here's what happens.
- After all combat damage has been assigned, all combat damage is dealt simultaneously.510.2 The damage event starts out as [7 damage dealt to player].
- The prevention effect (from Hallowed Healer) is applied, and the damage event becomes [5 damage dealt to player]. Since all the damage is being applied at once, you get to decide which damage is prevented.615.7
The results of the damage are processed now. Depending on your choices in step 2, one of several things could happen:
- If you blocked 2 damage from the Blighted Agents, the 5 damage all gets converted into loss of life119.3a. Both Latch Seeker and Tandem Lookout's abilities will trigger, and your opponent will draw 2 cards.
- If you blocked 2 damage from Tandem Lookout, 2 damage gets converted into poison counters119.3b and the other 3 is converted into loss of life119.3a. Latch Seeker's ability triggers, and your opponent draws 1 card. Tandem Lookout's ability does not trigger, since it did no damage.615.6
- If you blocked 2 damage from Latch Seeker, 2 damage gets converted into poison counters119.3b and the other 3 is converted into loss of life119.3a. Both Latch Seeker and Tandem Lookout's abilities will trigger, and your opponent will draw 2 cards.
- Many other options are possible - see below.
Finally, the damage event occurs.
Since you get to decide which damage is prevented, you get to decide which creature(s) don't deal any damage.
Furthermore, you can prevent any amount of damage from any sources in any combination - you could prevent 1 damage each from Latch Seeker and Tandem Lookout if you wanted. (In your particular case, it wouldn't be a good move - but there are conceivably situations where this would be advantageous.)