No, the game uses the highest defense value to determine whether or not you dodge the attack. It does not add them together. Think of the attack as doing LETHAL damage at MELEE range. Because you use parry you have a 15% chance to dodge the attack entirely.
Alternatively, let's pretend the Tsoo has a machine gun, whose bullets do LETHAL damage from a LONG range. You will still have a 15% chance to dodge because parry is giving you 15% LETHAL defense.
If a Circle of Thorns ghost joins him and uses Shadow Punch, which is NEGATIVE damage at MELEE range, you will again have a 15% chance to dodge thanks to the MELEE defense granted by parry.
If that same CoT Ghost uses Dark Blast, however, you will have a 0% chance to dodge because parry is only increasing LETHAL and MELEE defense.
Finally, let's pretend you are a Super Reflex scrapper who is using Focused Fighting to gain another 15% melee defense. Added with parry this puts your total defenses at 30% Melee, 15% Lethal. If the Tsoo attacks with the sword now, you will have a 30% chance to dodge, but if he attacks you with his machine gun you will only have a 15% chance to dodge because the machine gun is doing lethal damage from a range, which your melee defense cannot cover, so the game uses your lethal defense value instead.
None of this takes into account that enemies have a 50% chance to miss outright 'less they have an accuracy bonus power. The defense values are also being simplified for examples sake.
Thanks for long and great answer, you answered it perfectly, that I undertsand it, did I mention thankyou?
You thought THAT reply was long... try this:
http://boards.cityofheroes.com/showthread.php?t=121258
Arcanaville's guide defense is probably the most comprehensive and widely referenced guide on the topic that there is. It's very good, and very enlightening.
Say a Tsoo minion is after you with a sword, if you use something like parry, where you have defense to both his attack cause he uses lethal/melee, does the defense stack? Like 15 defense to melee + 15 defense to lethal = 30 defense to the attack. Is that true, this question has been bugging me for like awhile!