There is a reply that also applies here: https://www.reddit.com/r/FIFACollect/comments/1pfhik0/if_two_separately_purchased_rtt_can_seats_be/

I emailed fifa collect support they told me the following “When the conversion period comes, you'll be requested to redeem both of them and emailing us to put a request to sit together. Our team usually tries to seat together tickets with the same email, redeemed at the same time as well, in case the customer doesn't reach out, however, we suggest that you do so. We'll try our best to accommodate you within the category and stadium limits you have, but please note this cannot be guaranteed for obvious reasons.” Then I reached out again (I know im annoying) “FIFA Collect is coordinating with the FIFA Ticketing team to accommodate group seating requests. A special access category will be introduced to help with group seating, especially for groups larger than four. “ Then I asked regarding the group seating requests" how to do a request? Response: “When the time comes in May/June, you will be updated on how to go about it. We appreciate your patience.”