You could biamp with them, say use the 370 to drive the bass and the 350 to drive the tweeters. It's debateable how effective it is, but I'd say it's worth a shot and should be a lot more effective than just biwiring. You'd have to wire the pre out of one (this would become the pre amp) into the main in of the other, (becomes a power amp only) and the use the speaker posts on both to drive bass and trebble respectively.
I think they both have the same gain, so volume should be matched. There would be no power advantage, you'd just be splitting the speaker load across the two amps theoretically making it easier for them to drive the speakers.
If it makes no real difference you could sell one of the amps on, those NAD's are still respected today. I repaired my old C350 recently, I was pondering buying another second hand until I saw how much they sell for in less than great condition. So I ended up fixing mine for about £12 in parts, it's in my second system now.