Author your disk like normal inside of DVD Architect. Be sure to set in the properties that you will be using the 8.5GB disk. When you make your DVD, make sure to prepare it to your hard drive first in a folder. This will create the AUDIO_TS and VIDEO_TS folders. Use ImgBurn to burn the folders to a dual-layered disk. ImgBurn will suggest appropriate places for a layer break. Choose the best one (you can preview each layer break within ImgBrn) for your project and burn the disk. ImgBrn is a free program. Architect does not handle layer breaks well, that is why I say don't burn within Architect. Most of the time you'll end up with a coaster if you try and burn the dual-layer within Architect.
I have burnt quite a few DVD+R (Verbatim brand) discs with DVDA and never had a problem. The only issue is that DVDA seems to slightly overestimate how much disc space it will need (too conservative).
Just proceed as you would for a single layer disc. After DVDA has prepared the files prior to burning it will ask you how you want the layer break selected, and will indicate the suitable range for it.
I have ImgBurn and I like ImgBurn for some applications. But all of the projects that I create in DVD-A are burned with DVD-A without problems. This includes single and dual-layer discs using top quality media, and dual-layer blanks with the Sony and Memorex (gasp!) brands. I've never a burned a coaster, and have had no reported incompatibility or errors with set-top players.
I'm not a Sony fanboy, but I don't think it's reasonable to make a blanket statement about DVD-A's ability to handle layer breaks or suggest that the OP will end up with coasters. Sorry that your experience is different from mine, but I believe in giving the application the opportunity to do its job rather than blindly take alternative steps that may not be necessary, or even fail in their own right.