Going by my own experience, your plan below will be unsuccessful. When I queried the reason, the response is that when the lounges submit to PP for reimbursement, it would be in contravention with the T&C between the lounges and PP when you have one person with 2 separate PP cards on the same day at the same lounge.
Your solution is to use your PP card issued by Citi Premiermiles (Visa or AMEX) as the 2 free visits per calendar year can be used by cardholder and guest at the same time for lounge access.
In the good old days, if you were really nice and the lounge dragon was in a good mood, you could get away with it. That's because they would use those imprint device (like those for credit cards in ancient times for those of us old enough to remember), and can fudge anything they want.
Nowadays, they need to swipe the card on an electronic card reader and need to input everything properly so they cannot 'cheat' the system either.
Having said that, I think the Ambassador Lounges in Changi are abysmal and you aren't really missing much, even if just sitting at one of the chairs in the departure area, except maybe for the free soda.