It's ambiguous, but it makes sense that it wouldn't stack. Cast once, take CON; again and take STR, etc, etc. Second casting overwrites the first, etc, yes/no? Sources? ...and Yes, I know multiple enhancement bonuses of he same type won't stack.
It should work, similar to how the evil eye ability works. If a same spell has different effects then all of the effects should stay in play.
This assumes that one effect does not counter another effect of the same ability.
Example from an FAQ using "Evil Eye" and "Bestow Curse"
Witch, Evil Eye Hex: Can I use this hex more than once on a target?Yes. As long as you apply a different penalty with each use of the hex (AC, ability checks, attack rolls, saving throws, or skill checks), you can have multiple penalties on the same target. Applying the same hex penalty to a target just resets the duration to the most recent use of the hex.
Example: On round 1, you hex the target's AC. On round 2, you hex the target's attack rolls, so the target now has two evil eye hexes on it. On round 3, you hex the target's saving throws, so it now has three evil eye hexes on it. On round 4, you hex its AC again, resetting the duration of the AC-hex (which does not add an additional –2 penalty to its AC). The same thing would happen if two witches were using evil eye on the same target--as long as each evil eye hex applied a penalty to a different thing, they'd all apply.
This doesn't violate the general rule for stacking penalties--each evil eye effect is basically a different source, even though they stem from the evil eye hex (the evil eye hex is much like 5 separate weak hexes under a common umbrella). In the same way that multiple castings of bestow curse on the same target should stack as long as they do different things (penalize Strength, penalize Dex, penalize attack rolls, take no action, and so on), multiple uses of the evil eye hex stack as long as they're targeting different game statistics.