Lifetime Platinum Premier Elite from combined accounts not coming until Jan 2019 if you don't qualify under either standalone program . According to new FAQ posted, "If you earned Lifetime Platinum with Rewards, your new Lifetime Platinum Premier status should be reflected in your newly combined account. If you achieve 750 nights and 10 years Platinum Elite status by December 31, 2018, you will also earn Lifetime Platinum Premier status. That newly earned status will be reflected in your account in January 2019." http://status.marriott.com/faq/
I already have the 10+ years and 750+ nights combined, but am only Platinum Elite right now because my 14 platinum years are split between the programs. I don't understand why Marriott would delay it - particularly with my upcoming 3 week trip overseas, I would like to have the highest status possible to maximize points.