Any machine that will zig zag can use the double needle. You put two spools of thread up and then run them through separate disks on the tension. Then put one thread in each needle. Then sew. You want to be careful not to go to wide on your stitches. It makes very pretty decorations. My singer 401 accepts 2 needles without having to buy the joined needles. I can put any two needles in it and sew away. If one breaks I just get out another one.
I don't know anything about the curved needle - maybe goes to a blind hem machine?