Say I have 2 solar panels with slightly different ratings with ~36 VDC 8 A. I assume there is a voltage regulator with 18 VDC output, so I connect two 18 VDC voltage regulators and later try to bring it up back to 36 VDC by connecting the voltage regulators in series like batteries do. Is that possible? I'm trying to provide a steady input voltage to a single inverter.
This is possible if you use isolated DC/DC converters. If you use a LDO or something similar, the negative output is typically assumed to be at the same voltage as the negative input (and the two often share a pin on the regulator).
Since you want the negative output of your "upper" regulator to be connected to the positive output of your "lower" regulator, it needs to be isolated from the negative input.
If the panels match coarsely on current rating but not voltage, wire them in series. They will work at the spec of the smaller panel, which is no big deal if the difference is small.
If the panels match fairly precisely on voltage but not current, wire them in parallel. Use a charge controller appropriate for the resulting voltage.
Otherwise you'll need 2 charge controllers if you want reasonable performance.
Your trick with voltage regulators is not going to work. Controlling solar panels is complicated. The charge controller needs to interact with the solar panel's unique voltage/current behavior, or it's not going to get good power from the panel. The charge controller knows how to do that. So it needs to be given access to the solar panel directly without hardware in the way.
I wouldn't advise it.
If one of the solar panels is covered and doesn't produces power, you might have current externally "forced" through the second converter (which might suffer from it).
In an ideal world, even if you regulators survive and regulate, if they produce different current, you will be limited by the smaller of both.
So I would rather advise you to put either your solar panels in parallel, or the converters in parallel, rather than putting the converters in series