One note about multiple criticals, if you have 2 10% chances to critical, you don't have a 20% chance to critical.  You actually have an 18% chance for a single critical:

10% (chance of item1 to critical) * 90% (chance of item2 to NOT critical) * 2 (multiplier since you have two items which work indentically in this way, assuming you're just looking for a critical) = 18% chance for a critical.

Similarly, you have a 1% chance to activate both and do the 8x damage (10% for item1 and 10% for item2) = 1%.

Finally, if you're looking for any critical, you have a 19% chance to critical (combining single and double activations).  This would get more clear if you had, say, 10 critical hit items, but that's obviously not possible at the moment.

Just so you know, quick, geeky lesson in statistics for you.  If you did it your way, then rolling 10 10-sided dice would guarantee you one of every number (10% chance of each * 10 = 100%), which is clearly not the case.