Rather than visualize a single data set, you may want to go a step farther and explore the interrelationships betweens two more more scalar fields. This can be done, at the expense of visual complexity, by tagging data values with multiple independent attributes. Some possibilities are:
Of course, sharing attributes on sample points only works if both data sets were sampled at the same locations. If not, there will be two interacting sets of independent sample points, which will be hard to decipher visually, especially if the data set is dense.