When rolling for attack, I have two speaker bikes. Since the keyword "Impact 1" is a weapon keyword and not a unit keyword, I assume I would two "Impact 1" = Impact 2 if I have two speeder bikes. If this is true, do I have to roll them separately?

You don't have to roll them separately if you are shooting at the same target. And yes impact 1 is a weapon keyword not a unit keyword so it effects for each model shooting in the unit (in this case 2).

12 hours ago, Oberron said:

You don't have to roll them separately if you are shooting at the same target.

Just to be clear, if you are shooting the same weapon from both minis you are required to shoot them at the same target.

RRG pg.15: Attack Pool

Quote

An attack pool can consist of dice from different weapons,
but all weapons with an identical name must contribute
their dice to the same attack pool.

12 hours ago, NeonWolf said:

Just to be clear, if you are shooting the same weapon from both minis you are required to shoot them at the same target.

You're correct, but Oberron phrased it that way in case the target is in the front arc of one bike but the side arc of a second. You can still roll the Holdout Blaster for the second bike in the same pool with the Speeder Bike's laser (and even apply the Impact 1 to the die the Holdout Blaster rolled) so long as they shot the same target.