A process like this can take a long time, sometimes a generation or two. But it is best to have it take a while and wait for God's perfect timing so it gets done right rather than forcing it through because we think it suits our timetable and desires better. Considering how much time, energy, and other resources these processes consume it is best to be slow and methodical so we get the results that we want the first time around and avoid any unintended, negative results. This is what happened with the "Ecumenical Movement" of the 20th century in certain areas.
I can understand how some people would want to rush a process like this because it can make it seem as though the churches are "doing" something. That attitude reminds me of the rector of my old seminary who always had a major construction program going on - even if it wasn't needed and cost tons of money - because it conveyed "progress"... even if the seminary was mostly empty of students studying for the priesthood! Likewise, we have to have as our first priority spreading the good news of Jesus Christ and building the Kingdom of God. We need to engage in actions that will help us reach the lost and strengthen the Body of Christ and view mergers and unification programs as means to that end.
Personally, I see nothing wrong in simply spending an indeterminate number of years in fellowship and full intercommunion, while trying to work together on the ground and on other projects as much as we can. This has been happening in various ways between most of the continuing churches, so much so that we are already, for most practical purposes, one church. Once again, I applaud these two men for this letter and for their godly leadership, and may we offer prayers on their behalf, as well as on behalf of all of those involved in the process, and for all of the people of the two churches.