These are not the same as the interchangeable needles. There's too much difference in size to 'get gauge' by using the different sizes, that's not what they're for. The small one is generally around an 8 and the other an 11 or 13. The patterns I have you knit the RS row with the little one and purl (or do the WS row) with the larger one. It creates a sort of lacey effect.
sue- knitting heretic