When I built up our Calfee several years ago I decided to go with CX-Ray spokes because of a tiny perceived advantage in weight and aerodynamics and maybe some bling.
Since then we have been breaking spokes on the rear wheel on a occasional basis, maybe 5 total.
They always break when we are out the saddle standing, and they have broken in different places.
Recently we had two break in a span of weeks and I am now going to rebuild the wheel with DB14 spokes.
I will admit to being an amateur wheelbuilder but have been doing it for quite some time and consider myself pretty good at it.
I might have done something not entirely correct on this wheel build.
I did notice that the spokes enter the nipple at an angle and I should have formed the spokes to reduce this angle.
In any case I think the DB14 spokes are probably a better option for a rear tandem wheel.

Thanks for the information. Many may benefit from your decision to share the result. There are often glowing reports posted about new equipment with little followup and I often wonder how many are later are silently abandoned.

With my Enve rims I was limited to 28 spokes for my standard front/rear disc wheels. For this reason I chose the 65 mm rims (to add wheel strength, and make the spokes shorter), and 5.9 gram/spoke DT Competition spokes, instead of 4.3 gram/spoke DT Aerolites.

In two years and about 7,000 miles, we haven't broken any spokes, or had any spoke problems whatsoever. And they are still lighter and more aero than Rolf's.

What hubs did you use?
Any non spoke issues?

I have CX Rays on 3 bikes (one of them is the tandem). I have never had a problem with them. I have broken a couple alloy nipples (not the spoke itself), and since switched back to brass.

More here. The front hub is a DT 240s and the rear is the 135 mm DT 240s disc brake. The latter is a mountain hub. 135 mm OLD gives a lot more choices in hubs. I could have used a DT 350s rear disc, and have it a bit more robust. So too could I have used the DT Swiss 540 rear disc tandem in 135 mm, but it has 36 or 40 holes, and I needed only 28. The primary reason for the DT 240s rear disc is that I could use a Campagnolo freewheel, which are not usually made for 135 mm disc hubs.

BTW, Enve just realeased their new Smart System aero clinchers, which look to be even better for tandem use.

Non-spoke issues? Uh...yes.

• My rear tire violently blew, and this disrupted the carbon of the rim. I discussed that here. Enve warrantied the rim, and I got their newer better version (better heat management). Enve also replaced the front rim, which they found to have an uneven brake track. The generous response from Enve left me more of a fan than I'd have been if the rim hadn't delaminiated.
  .
• Just recently, my bearings were shot, and my freehub was cracked. I might have cracked the freehub with my hamhanded overtightening on a cassette change, and perhaps this contributed to the bearings. I don't know whether or not this proves the DT 240s to be insufficiently robust for tandem demands.

I have them on my single 20F 24R. I also built those wheels and have not broken any spokes and they have stayed perfectly straight.
Your tandem wheel must be built really well, I know you are putting more stress on it than we are.
Brass nipples always.

Yeah the alloy nipples were recommended by a builder for my single bike. Never again.

Actually, I only have the CX-rays in the front of the tandem. Used regular DT double butted (14-15-14 IIRC) in the rear because of the disc brake. 32 CX-rays up front (3x) and 36 of the DTs in the rear (3x). I figured the CX-rays wouldn't matter too much in the rear aero-wise. But man those other spokes are heavier!

The wheels were built by Precision Tandems.

I have not much problem with the front, only one broken spoke that I remember.
Hoping the DB14s and my improved wheel building skills will result in a more reliable rear wheel.
Yeah when you hold them in your hand the CX Rays feel really light, but it really amounts to the weight of some pocket change.