Welcome aboard to CD Freaks
You can install both cards in the PC and use them independently or both at the same time (e.g. watch one & record the other). Depending on the DVB tuner software you use, the channels can be grouped in two separate lists (e.g. one for DVB-T and another DVB-S/S2) or merged together as one long list. In DVBViewer, you can create a group for your favourite channels between both sources, so you can switch between channels as if they are all from the same source.
If you have not bought any card yet, another option that might be worth considering is an all-in-one DVB-T and DVB-S/S2 tuner, which will free up a PCI slot. These have separate terrestrial and satellite inputs and are seen as two separate devices in the OS. The combined card I'm using is the HVR-4000. The only drawback with this card (and possibly other combined tuners) is that both DVB-S and DVB-T cannot be tuned simultaneously; for example, it's not possible to record or stream DVB-S while watching a DVB-T channel at the same time.
The following gives an example of how both tuner types show up in DVB Viewer:
Great to hear that you have no problem picking up the Freeview free to air channels in France. I was so happy when the BBC and the other national TV providers finally ended most of their deals with Sky to go free to air. Beforehand, the only way to pick up the BBC channels on satellite was to get Sky's proprietary receiver and a Freesat viewing card, which is difficult to get for anyone who lived outside the UK.