I think there are a few questions one should generally consider when asking questions like this.

1) Are there any established sartorial norms for this kind of thing?
2) Are there any historical precedents to consider?
3) Would it look good when worn?

In the case of wearing a plaid of one tartan and a kilt of another, the fairly obvious answer is that the two are generally worn of the same material. One would not have to look long, however, to find examples of the plaid and kilt being of the same tartan but different colors (Atholl Highlanders, for example).

Are there historical precedents? The previous poster mentioned that in the eighteenth century it was not uncommon at all to mix tartans in one outfit. However, as he himself stated, this was in an era before named tartans, and tartans were worn with a completely different mindset than today. I would also point out that the modern use of the plaid (in its many various forms) is meant to echo the older mode of dress, the feilidh-mor, in which case the plaid from knee to shoulder was of one piece. For this reason, the plaid in modern Highland dress would generally match the kilt.

Third, would it look good? Depends on the two tartans in question, I suppose. And the rest of the outfit. And how you carry yourself. This is, by and large, a matter of opinion.

However, I would advise to steer clear of rules that seem to have no real reason for existing, such as "it's ok to mix tartans, except for dress tartans." Why? If it's ok to mix tartans, why should dress tartans be any different than hunting tartans or regimental tartans or any other?