AB40k Maintainers

Using AB 3.4 and the Rel 1 of the 6th Ed data files I seem to be having an issue configuring a Chaos Dread. If I give it 2 Dread CCWs I can only replace on of the TL Bolters with a Heavy Flamer. When I try to replace the second TL Bolter I get a Dred Option Error. As far as I can tell this is legal according to the Codex. Am I missing something, or am I doing something wrong?

I may need to go though the FAQ, but at the moment the CSM codex states on page 95, "a single twin-linked bolter may be upgraded to a heavy flamer..."