Q306. Create custom labels based on two fields for a polygon layer.
The following is from How to Make Multi-Line Labels in QGIS. That's more than just using two fields, but the two fields part is the same in both cases.
Sometimes we want to label features like this:
We can do it by building the label expression using a tool we find under Layer>Label
We click "Label this layer" and then rather than selecting a field, we click on the Expression Builder
We build the expression for the label string, using fields and constants and operators and conversion functions. Note that the concatenation operator in QGIS is ||. We want our label to be on two lines so we insert a character that we will tell QGIS to treat like a "new line" signal. In this case we will use "/"
Back in the main dialog we can specify the new line character and play around with other stylizing options.