I currently hold Plat Charge and Explorer, and I signed up for them in that order. When my Explorer card activated ~4 months ago, it was linked to the Plat Charge card and I didn't receive the signup bonus. I had them unlinked, then upon re-spending $1500, the bonus appeared. Great success. I am now trying to re-link them. Just as a point of feedback, I called at 7pm on a weeknight and hit a US sounding call centre, who denied the request. I will try at 6am as others have indicated success with
On a more worrying note. I have a friend who held the Velocity Platinum card, cancelled, then 3 months later signed up for Explorer with my referral. He hit the min spend a month ago and didn't receive the bonus. He called Amex about it, and they stated because he has already held a credit card with them recently, he wasn't eligible for the bonus. This is interesting as we know that is the policy but I have never heard of it happening in practice before. Are amex really clamping down on enforcing the one bonus per credit card, one bonus per charge card rule?