Say I'm a rogue, and I'm trying to activate a scroll of magic missile with a CL 1. Let's assume that I've already successfully deciphered the scroll, and that my Intelligence score is sufficiently high.
Do I have to make a single UMD check to activate the scroll?
Or do I need two UMD checks -- one to emulate a class feature (i.e., casting spells), and one to activate the scroll?
You need only make one UMD check in that situation
Use a Scroll: Normally, to cast a spell from a scroll, you must have the scroll's spell on your class spell list. Use Magic Device allows you to use a scroll as if you had a particular spell on your class spell list. The DC is equal to 20 + the caster level of the spell you are trying to cast from the scroll. In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability. If you don't have a sufficient score in that ability, you must emulate the ability score with a separate Use Magic Device check. This use of the skill also applies to other spell completion magic items.