Do I use a twin needle with my narrow pintucking foot on my Elna quilter's Dream 2 sewing machine?

If you want to sew pin tucks, then yes, you could use that foot providing the hole is large enough for the two needles to pass through. Twin needles come in 4mm gap and 2.2mm gap so just check that it will fit. When using a twin needle, there is a lot of heat created so it is always important so sew slowly, going too fast will cause the needles to break.

Also, you should check on the twin needle type, modern machines now using a twin which is slightly offset, the block colour is usually a dark brown/red colour. Your machine may have come with a twin needle in the needle selection so take a good look at it and see if it is an offset one, then make sure you buy replacements of the same type.

THe pin tucking foot allows you to keep rows of pintucks lined up, it will have grooves in the base that you can run the previously stitched pintuck through to keep even distances. Best on lightweight fabrics like cottons, lawns, batiste etc. You'd probably go for the narrow 2.2mm twin needle to get a nice fine tuck. I like to press each seam after stitching too to give a sharp tuck.