ThankYou now allows transfers, but the points expire 90 days after which means you should not transfer until ready to redeem:
https://www.thankyou.com/transferPoints.jspx
In the USA, there is no published provision to transfer points or combine American Express Membership Rewards accounts, but you may find a customer service agent who will do this for you on an exception basis.
Thanks for the Thank You heads up - good call