On the level Halo, it is possible to wield two Spartan Lasers at once (not to be confused with dual wielding). First, proceed up the tower with a rocket launcher. When you reach the top, do not kill all any Flood or Sgt. Johnson will come down and cause the glitch to not work. Blast him off the cliff with the rocket launcher. Once he falls down the cliff, get the Spartan Laser he is holding. When he gets up, he will be wielding the weapon you exchanged for the laser. Kill Johnson again. when he gets up, he will magically holds a new Spartan Laser. Kill him only with meleeing. Then, use your non-laser weapon to swap for it. And Hey Presto! You have two Spartan Lasers! To check, reppeatedly press the Y button to check your weapons. There is a fun animation when you switch between the two Spartan Lasers.
Unknown Laser
There is also another way to obtain 2 Lasers, but it is more complicated and requires you to complete Cortana (Level). First, on Normal difficulty (or higher), start on the beggining of the level Cortana. Find Cortana, and leave that room. Then, while in the room where you destroy the generators, try to jump on a ledge jutting out that is about middle height. After a few tries, (grenade jump and cowbell skull possibly needed) you should be on the slanted ledge. Feel free to snipe some flood while you're here. When you're done, look around the slanted ledge, and eventually you should find a pile of bones near the edge. Among those bones is the Unknown Skull, take with you until the end of the level, and in the level Halo, you should have a Spartan laser as the first weapon, and whatever weapon you had backpacked in the previous level, Cortana. You could kill Johnson, or you could just take the laser he gives you in the battle against 343 Guilty Spark.