the hearts; then turning knitting back to right side, which leaves the heart on the left needle, and needing to be slipped to the right needle; then slipping the heart stiches onto the right needle, carefully undoing and re-knitting every other stitch so that I could hook the main stripe colour yarn along the back. Fiddly, but worked well, and only had to be done on three rows.
I am a confident knitter, so was not daunted by the prospect of fiddling this way. However, an alternative way to achieve the same visual result would be to knit the wrist warmers without the hearts, then darn them on afterwards. Thanks to bluestockingstitching for this suggestion.