Do you mean like a travelling cable? You'd slip two knits to a cable needle and hold in front, purl one from the left needle and then knit the two from the cable needle.
__________________
"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
Yes, that'll do it for a left leaning cable. For the right-leaning, you hold a purl to the back on a cable needle, knit 2, and then purl the stitch from the cable needle.
__________________
"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