I believe how magic resistance working on enchantments was more like, "if your comfortable with it and want it, it last longer." So if you WANT an enchant but it make you uncomfortable, it would still be resisted at the normal rate instead of a reduced one.
Having two enchantments up does not reduce the duration of either. Enchantments don't have upkeep as far as I am aware, once it's cast it's there till it's resisted away.
Pandora made it sound like it's something any rookie might do, not something specific to this situation but I guess the enlarging enchantment and the shrinking enchantment could interact some how.
