Jim & Metrosheriff are 100% correct that an ATP can NOT instruct in this scenario. The only way an ATP can instruct is if he has been "blessed" as an Instructor for his 121 or 135 company and then he can only instruct for the 121 or 135 carrier.
Now as far as logging SIC in a Citation 2, what kind of Citation 2 is it, a 550, S550, 551, or Bravo? Guess what only 1 of these Citation 2's is certified as for Single Pilot.
If you are flying a 550, S550, or Bravo log every minute of it as SIC end of discussion. Just make sure you receive a 61.55 checkout from the PIC as they are certified for TWO pilots. Just look up the requirements in the FAR's for 61.55; basically 3 landings some SE stuff and know the airplane, just make sure the PIC signs your logbook or gives you some kind of paperwork (by the way the ATP is NOT "instructing" in this checkout he is simply acting as PIC and the checkoutee is acting as SIC as provided by 61.55f).
Now the PIC can fly the 550, S550, or Bravo Single Pilot. If he receives a SINGLE PILOT WAIVER, this is an additional check ride that the PIC must take every year in addition to his normal 61.58 check ride. According to FS and SF the Single Pilot Waiver is the hardest check ride they can give; the reason for this is because the candidate has to demonstrate a circle to the right and a raw data approach off the copilot’s instruments in addition to the normal check ride maneuvers. Also the Initial Single Pilot Waiver has to be done in an airplane (recurrent can do it in the sim). Consequently, very few Citation pilots have and maintain a Single Pilot Waiver. Additionally the Single Pilot Waiver costs an additional $9,000 to at FS and insurance typically goes up about $15,000, i.e. it's cheaper to hire a copilot!
Anyway even if he has a Single Pilot Waiver, once he puts a qualified SIC (61.55 checkout) onboard he is no longer exercising the privileges of the waiver and you can log every minute of it as SIC!
Now for the complicated scenario the 551! I have talked to numerous Citation DE and FAA Citation Inspectors and they all said they will accept all SIC time logged in a 551, as long as you have a 61.55 checkout. Here's the reason for the confusion, people believe that it is certified for 1 Pilot, it is NOT. The Flight Manual states that the airplane is certified for 2 Pilots OR 1 Pilot and boom mike, autopilot, etc. Once a qualified PIC and SIC are onboard it is certified for TWO pilots, so log all the SIC time you want.
So just make sure you get a 61.55 checkout and document it, and make a copy of the limitations sections of the flight manual (the page that states 2 pilots). Just in case anybody ever questions you about the time you can just whip it out and shut them up.
If they're VERY, very knowledgeable and still maintain that it's Single Pilot, just tell them the PIC didn't like using boom mikes and you guys used the overhead speaker. This will shut up even the most proficient Citation SP naysayer, because in order to operate a 551 SP the type requires a boom mike, without a boom mike the 551 is not allowed to be flown SP an SIC is now required by even the strictest regulations!
In other words you can LEGALLY log every minute of Citation time, enjoy flying it! You'll miss the Falcon; a Falcon's a much better "pilot's airplane".
Sorry to be so long winded.
