What happened to the second kira? After Light and Misa were cleared, everyone seemed to just forget about that there's more than one kira. What was up with that? I seem to recall Light instructed Misa to make a response to the task force's kira video, then L deduced that the two kiras have met, then Light and Misa were confined, then Soichiro fake shoots them and then they try to catch Higuchi. While trying to catch Higuchi and thereafter, has everyone just forgotten that there's more than one kira? Did anyone find it suspicious that there seems to have been only one kira around? Maybe they thought that the two kiras were acting as one?

Based on a comment I made here: Why didn't the Task Force capture Misa Amane?
There's nothing to expect from the Task Force. Their main brains were L, Light and Near. 

L proposed that Misa is the second Kira, but he was proved wrong because of the fake shot scene by Light's father. So inside the Task Force's mind, the claim that Misa is the second Kira became false, thus the claim of second Kira was probably forgotten too by the Task Force.

After Higuchi's capture part, L still didn't forget his claim about the second Kira and Kira = Light, because L wanted to test the truth of 13-day rules. Unfortunately he didn't live long. 

Task Force is just a group of subordinates who receive the orders and execute the orders. They are not the leader of the team. They just follow their leader's instruction. They didn't have insight like L and Near, plus they believes in Light, so they just did their job according to Light's instructions. Even though Aizawa doubted Light after Near emerges, he still didn't have enough brain to prove Light = Kira, and he just had doubt, not confirmation. 

As part of a team, Japanese values teamwork and harmony in a team, so they follow their leader. As for the guy who doubted Light, he chose to follow Near's instruction.