A simple scenario I thought of while learning how to GM Pathfinder, if a wizard takes the Still Spell feat alongside Silent Spell, will the effects be combined? And will the drawback on both being the spell takes a slot one level higher also combine, making it two levels higher?
Yes, you can. Their effects combine, and the level gain is cumulative. See also:
Multiple Metamagic Feats on a Spell: A spellcaster can apply multiple metamagic feats to a single spell. Changes to its level are cumulative. You can’t apply the same metamagic feat more than once to a single spell.