Ok. So there is a thread VERY similar to this already but the thread was made for xbox and makes it a little harder on us PS3 users.
so i mastered the glitch and figured out the easiest way to do it and the fastest way. ok so here goes.

1. Log onto Elite on your PC and Set up your 1st Class with the TWO attachment setting but only add 1 attachment.
   
2. SEND changes to game and accept. this will now make it where both your PC and PS3 are on the same page.
   
3. Now go back to the PC and select the first class again and this time change the proficiency
   
4. SEND the changes to the game.
   
5. Now on your PS3 back your game up all the way to the main menu
   
6. now click back into multiplayer and click online and as soon as it stops looking for servers click create class and then primary and then the attachments and hover on the second attachment you'd like on your weapon
   
7. now the ELITE message should pop up Accept it and back out DO NOT GO ANY FURTHER FORWARD (it removes the proficiency) and poof you should have 2 Attachments with any proficiency.
   
8. 
   

I do not take credit for this glitch, and im not exactly sure who to give credit to.? im just trying to help explain it better for the people that would like to do it.