Preciesly what I did at Kathmandu last night. Split payment across eight cards to maximise cash back.I think you've just done two transactions to get to the spend threshold for one credit.
In this 'game', a split transaction is where you pay for your purchase with two or more Amex cards to trigger the credit on each card.
I also got the same offers on my platinum card.Received a welcome offer of spend $20 at PayPal and get $5 back (up to 2 times).
This is in addition to the following welcome offers I have received in the first 5 weeks of a new card:
Spend $10 get $5 back at Woolworths
Spend $30 and get $5 back at Uber (up to 2 times)