Welcome to FT janman74!

You can't combine miles between accounts. As stated, all you can do is pay to transfer up to 15,000 miles a year between accounts which isn't much of a deal.

I try and build up miles in just one account because that works better for us. As stated, booking one-way awards is the way to go if you don't have enough miles in one account for a round trip. You just won't be able to be on the same itinerary that way. If you take this route, call and have your reservations linked.