Silence = -4 to Notice against Sounds, but what about Notice against Stealth in general?

Silence gives anyone who tries to hear you -4 to their Notice roll (or an auto-fail with a Raise), but how much does this spell influence your Stealth roll where you also have to stay out of sight?

I am planning on combining Silence (-4/auto-fail to Notice) and Invisibility (-4/-6 to Notice), but I don’t know how to combine these numbers. Without the auto-fail I would’ve just added them up, but the auto-fail gives me the impression that it was intended that I don’t do that in general with these spells.