Good question.

One thing to note: the difference between varying the proportions so the two appear the same, versus look clearly different. From your other thread I assume you're looking for the former, although both scenarios are worth learning about.

hhp

I'm looking for an example of them appearing clearly different, but, as you said, both sceneraios are interesting.

The concept of mixed sans/serif families is fairly new (eg something that has started in the last decades). Earlier examples are in the line of what Frutiger did: using an existing sans or serif family as a starting point for a variant serif or sans. If I recall correctly the block serif Serifa shares a lot of basic forms with Univers.
Another example of this procedure, but maybe more controlled is what Martin Majoor did with Scala. He first did the sans and the serif a couple of years later.

I don’t think you will find a lot of examples.