Combining / superimposing data series with different date ranges

Not much to go on. It would be helpful to upload a sample workbook with some sample data and the charts you are currently using. Then we can see better about how to combine the two charts into one.

My first thought is that this will be fairly easy with a scatter chart. Each data series can have its own X-values range, and Excel should easily plot the points where they should be. You don't say what chart type you are currently using. Would a scatter chart be suitable?

I see two options, and I am not sure which will be more "confusing" for you:

1) Use a scatter chart and assign the appropriate x value range for each series. I'm not sure what is confusing about this. I expect it will be tedious to go through each data series assigning x value ranges, but it should be easy.
2) Combine all tables (your sample only has two tables, are there more?) into a single table. In this table the x values column will combine and/or merge the time stamps from the source tables. This table will also have a lot of "blank" entries where a given data series does not have an entry corresponding to a given time stamp. Then you can build your scatter chart (I would still use a scatter chart so that the times are treated as times and not evenly spaced text) from this table, using the same range for the x values of all series. You may want to review this (https://peltiertech.com/mind-the-gap...g-empty-cells/ ) to understand how Excel will treat all those blank cells -- especially if you end up using formulas to build this merged table. This approach avoids the tedium associated with editing each data series, but it adds the tedium of building another table.
2a) Get and Transform might be able to easily merge these tables, but I am not familiar with this utility.

In either case, I would use a scatter plot so that the times are real time values and not just text categories, just in case it makes a difference. I am not sure which will be more confusing.

You have a lot of data in those tables. Trying to put too much information into a single chart will cause confusion whichever way you choose to do this. If your use of confusing alludes to getting too much data into a chart, you may want to consider what you are trying to show with these charts and figure out the minimum data to put into the chart that will communicate what you want.

There are probably a few different ways to get a sample of points from a large table. Something as simple as =INDEX(main_table,ROW(A1)*10-9,column#) would sample every ten rows, for example.