I asked LastPass support about both the Xmarks+LastPass Premium (Premium combo) as well as the 64-bit Xmarks iOS app. Here are their replies:
Question: Given the new pricing for LastPass Premium (and the new LastPass Family), how will the reduced payment for the combination of Xmarks and LastPass work for me with LastPass's new $24 price. Before this change in January I paid for a year of both Xmarks and LastPass for $20 total instead of $12 and $12 or $24. What is the new policy for a combined subscription?
LastPass Support's Answer: Premium services will differ from Family services. Therefore, the two services will overlap in their functionality in certain regards (i.e. one-to-many sharing, Emergency Access, etc.). Therefore, it is recommended that either Premium or Families is active, but not both. Xmarks can be purchased in the future a-la-carte or using the Premium combo which you currently have.
Question: I need a 64-bit version for Xmarks for my iPhone and iPad. Without this there is no reason for me to continue my Xmarks subscription. When can I get this before iOS 11 is released in a month or two?
LastPass Support's Answer: As a beta operating system, iOS 11 is not officially supported. However, our team is aware and prepared for the launch date.
Looks like there will be a 64-bit iOS version of Xmarks by iOS 11's launch.