Here's where I ended up.

There's no mechanism for combining two series with different sampling intervals. Theory requires the same interval.

Re-sampling is fraught with problems, not least of which is adding artifacts. Re-sampling needs to be done very, very carefully.

In the end, I did some conditional re-sampling:

• If the series with the smaller interval has a range less than the interval of the larger series, I approximated the smaller series as a delta function
• If the series with the smaller interval has a range equal to the larger interval or more, I upsampled the larger interval to the smaller. It's not a great a solution, but ti works.