Well it's certainly not a bad idea, and I'm sure it would probably sound pretty good. You can do better with less $$$ though. The SH-50 and SH-60 to me are the ideal Synergy horns. Yeah, you can use the coax, but the alignment of the mid/treble is fixed and I'm not so sure how smooth the transition is between the 2. Also, with an SH-50 type you're looking at a 1" exit CD vs the 1.4" on the coax so the 1" is going to have better directivity a bit higher. I believe Danley himself has said the coax drivers aren't ideal.
The nice thing about a Synergy with a 1" cd and 3 or 4 4" or 5" closed back mids is you can get the mid ports designed and placed correctly for the thing to act as a point source, at any axis you care to measure. If you have the crossover chops (or FIR capabilities) you can get it to have flat phase at any angle as well without any wrap through the crossover. I'm not so sure the coax can do that. The closed back mids vs the coax can also get a bit lower, allowing you to use larger bass drivers on the horn if you wish to go that route.
Look at the cost of the coax vs say a DNA 360 and 4 suitable mid drivers (which admittedly are hard to find if you don't know what you're looking for or aren't following an established design). The coax are what, $700 a piece? The 1" cd and 4 mids might put you around $200 per channel depending on what you find for mids and I would argue would give better performance overall.
That said I don't want to knock the coax units as I've never heard them. There's several here using them to good effect, it's just not the approach I would take and I think a Synergy horn is better suited going the more traditional route.