There are some restrictions to the attachment combinations (additionally to those which are also present on primary weapons): Akimbo, Tactical Knife and Laser Sight all conflict with each other. In addition, Akimbo cannot be used with any kind of optics, nor with the Parabolic Microphone.
Note that on weapons that are Akimbo by default (SAC3 and XMG), the use of Laser Sight or Parabolic Microphone is allowed.
Secondary Gunfighter returns in Call of Duty: Black Ops III. It is mostly the same as in Call of Duty: Black Ops II except optics are counted as separate and do not count as an attachment, so with Secondary Gunfighter the player can have two attachments along with an optic if they have enough Allocation Points.