Double butted spokes are definitely an excellent choice for long-lasting wheels; far better than straight gauge. Not sure what Wheelsmith 13's are, but DB14 or even DB15 would be more than adequate.
As for rims, I've put Velocity rims on all three of our tandems -- Deep-V's on the road bikes and the Aeroheat AT on the off-road tandem. Excellent rims that build up quite nicely. In fact, I just rebuilt a 700c Aerohead OC this past week and it came up to tension and trued as easily as any Mavic rim that I've built up.
Mavic's "tandem" rims:
- The 700c T520 comes in 32h, 36h and 40h
- The 26" X517, X221 & X521 come in 32h and 36h.
There are also some other, older models of Mavic rims available from wheel builders with different designations, e.g., T517, etc...