Basically would work the same as GS crafting, combining two shards together to make one with dual effects. There would be no special added benefit to doing so, except to have two abilities on one shard.
No more boring crafted items with just a single prefix and suffix! With this new rule you could have up to two prefixes and two suffixes on one item, with a total of up to four effects per crafted item.
Same rules for ML and maximum enchantment value would apply, so you could only fit so many things on an item, nothing changes here, you still have to pick and choose. You can fit up to the item's maximum enchant level (currently +15)
With dual-affinity crafting, you get a slightly expanded selection and would be able to get much more creative with with the gear you craft.
Helpful suggestions/comments welcome.