You know, I was thinking the same thing. One of the SBBEG's (Sorta-BBEG) in the game I'm DM'ing is a sorcer who specializes in battlefield control. Solid Fog, Cloudkill, Wall of Sand, Waves of Exhaustion...
I'd say the 'Fog' spells wouldn't overlap. The highest level one would displace' the lower level ones... and when one ended the other would blossom out to it's full radius immediately (if a fixed radius fog) or expand to fill the space at it's normal rate (i.e. Cloudkill). I see a parallel to Darkness + Deeper Darkness... one trumps the other since the effect is the same, only 'more'.
I'd also say other non-Fog spells could overlap... Solid Fog + Waves of Exhaustion is a good combo. Blade Barrier would work fine, IMO... however, tossing a Fireball in the middle of a low-level Fog spell... would that equal the force of a Gust of Wind and dissipate the fog? Would people inside a Solid Fog take less bludgeoning damage from Ice Storm because of the 'slow fall' effect? Mmmmaybe. However, that's house-rule territory.
I sort of miss things like AD&D Fireball's unpredictible behavior within dungeon corridors. Having weird spell interactions - and making some interesting calls that allow for creativity - make for a more interesting game IMO.