You refer in your second post (http://forum.step.esa.int/t/reprojection/5673/1) to SMOS Level 3 data. So I assume you are using Level 3 data here too.
Unfortunately Level 3 SMOS data is not supported. It uses specific grid (EASE 2.0) which can not be read properly at the moment. There is a shift in the geo-coordinates. You can see it if you add the world map background.
In general bands of different products can be used as layer in one view, if they use the same coordinate reference System. E.g WGS84 or UTM etc.
So you would need to reproject both products. But as the EASE 2.0 grid is not supported you will always have a shift.
There is already a thread about it: