The triple cranks have their middle rings designed for better shifting (with the aids I mentioned) for the granny to middle shift. The double ringed cranks don't need these aids as there's no shift up and onto the 39T ring. That the shop either didn't know this or didn't explain it to you speaks to that shop's knowledge base or their lack of communication motives. We interchange a number of brands of rings on various cranks with little issues when the spec are matched right. We do like to use the same brand rings whenever as the first choice but find some brands shift better then others, even when all have their shift aids. Shimano is still what we call the golden standard. The race face rings shift very well too (and what I personally use on a number of my bikes including our tandem). Dimension and Origin 8 are house brands from major wholesale suppliers and also shift well. FSA is among what we feel are the worst shifting.

Tell your shop that QBP shows both FSA and Shimano 39T 130 bolt pattern rings in stock right now. Andy