This question seems easy at surface, but there are some things to consider here.

Does your meters report data at same time? If not, how large is a time difference between them. I assume that if it is anything grater than 1 minute, you can't just do value_1-value_2 and call it a day.

What I have done for similar case is calculating the difference every 10 minutes by using equation of line. You need a rule chain that will take telemetry data of both devices, go over that data in chunks of 10 minutes, make equation of equation of line thru 2 points, and then use those equations to approximate the values of those two devices at the same time. Only then you will get semi-ok results.

If you really have some devices that report to you at few seconds periods, on event of new msg from device in root rule chain you can pull last value of other device and do your calculation. This will most certainly produce some saw like line on chart.