I have two choropleth of the same area but with different concentration data and I need to overlap them. My aim is to highlight the areas of intersection between the points with the highest concentration (I use a quantile interval). It is probably only a matter of mergers, but I made a couple of attemps and the graphics quality has suffered (e.g., multiply, burn). Any suggestions? Thanks in advance.
As Chris W says, I think that you are trying to build a bivariate representation of your data (not spatially intersecting).
Personnally, I find this easier to do in raster. Use the intensity in red for one layer and the intensity in cyan for the other layer. Black will mean low everywhere, white large everywhere, and if you can see a color it indicates that one of the band has larger values than the other band.
to convert your vector to raster (see here for details)
then create a composite image with
gdal_merge.bat -separate -of GTiff -o output.tif input1.tif input2.tif input2.tif
you can now add the image and see how it looks like.