I'm assuming that one tail is too short. You can add yarn, but you don't want to knot it.
I don't know how it will show too much for 40 stitches, but I've switched to knitted cast on if I'm a few stitches short--only used one strand.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry