So what Dan is explaining, you could use:
Con("AlnusSpp" + "BetulaSpp" > 20, "AlnusSpp")
... in case you want to create a new raster with the original values of "AlnusSpp" where the sum of "AlnusSpp" and "BetulaSpp" is higher than 20 (rest of the pixels in this case will be assigned NoData)
And you can use:
Con("AlnusSpp" + "BetulaSpp" > 20,"BetulaSpp")
... to create the same type of raster but now with the original values of "BetulaSpp" when the sum of the two raster is higher than 20 (rest of cells will be NoData)
Or you can use something like this:
Con("AlnusSpp" + "BetulaSpp" > 20,"AlnusSpp" + "BetulaSpp")
... when you want to create a raster with the sum of the values of the two raster when the sum is higher than 20 (rest of cells will be NoData)