The Aish Rabbi Replies

The general rule is that if both of the garments can be removed without first removing the other one, they do not combine to be Shatnez. If however, for example, the shirt cannot possibly be removed without first removing the sweater, they do combine. Nevertheless, it is best to avoid such combinations, even if one can somehow remove the inner garment without the first removing the outer. (Sources: Shulchan Aruch Y.D. 300:4, Pitchei Teshuvah 6.)