my combat system ... any ideas or is it ok?

What kind of game is this? A tabletop wargame, strategy board game, etc.? As much as I love rolling dice, a system like this is best suited for skirmish conflicts of about a dozen or so pieces on each side. A handful of dice can become unwieldy once you move to a larger scale, unless it represents force of numbers, like rolling forty dice for a unit of dark elf crossbowmen in Warhammer. Games with a lot of dice also trend toward a smaller player base. For example, Risk, as a traditional mass market game, limits itself to 3 dice per roll at most.

I'm too lazy to work it out here, but have you done any of the math for calculating odds on a percentile basis?

Combining defence points.
My advice: No.

Now, I don't want to hijack the core of your game, nor do I want you to feel that way. So don't use the info in the PM that I am going to send you. But in there I will give my suggestion.

More fun for both players to roll dice, and with the current system, if a player ever gains more defense than the attacker has strength, it sounds like they couldn't even attack them.

Technically, people weren't playing it. WH40K was accounting for 50% of GW's revenue stream, while Warhammer occupied massive shelf space but was in the single digits percentage-wise for sales, after licensing revenue, etc. That's why they destroyed the game world in their End Times product line, officially axed the buckets of dice Warhammer game, and just now released Age of Sigmar with its 4-page rulebook, completely different mechanics, new setting, and skirmish-oriented scale. The Warhammer game of the past thirty years is effectively gone because it didn't resonate with younger players.

The market has shifted toward skirmish scale games with fewer dice rolls, less tables, and free rulebooks supplemented by unit cards printed with special rules. Also, I go, you go turn structures are considered very passe.

Good to hear that it is on an individual basis. Every one here previously thought that you where adding up the defence points.

Did you read my PM by the way?

Those COOP units are a good idea. Somehow I see this happening as medics.

I think the idea of both players rolling is a good one - if the defender takes damage, he doesn't have the fatalistic mentality of, "Well, there's nothing I could do to prevent this," and instead gains the thrill of, "I have a chance to do something about my circumstance, for good or ill." This is encouraging to players, and reduces the chance that someone cries, "The game is broken because X is too powerful."

At the same time, the Colonel has a point - more rolling means more time taken up, more math (which can go wonky when it's done too late at night), and can give quite a bit of variety in the results, so I think there's a few fundamental questions we should ask regarding attack and defense:

1) Static Value: Do we want to have a static value for the attack and/or defense value? Hypothetically if someone were to fire a weapon with an Attack of 3 and someone else were to fire a weapon (call it a cannon on a tank) with an Attack of 8, if the A3 rolled really well and the A8 rolled really poorly we 'd be seeing a guy with (say) a rifle outperforming a guy with a cannon in regards to damage. Could be believable (the shell was not centered right, the bullet hit just the right spot, etc.), but it's a question we'll want to consider.

Similarly, do we want defense to be static? If the guy rolling Defense 2 rolls well and the guy rolling Defense 7 rolls very poorly, could the D2 resist better than the D7? Sometimes yes (similar reasons mentioned above), but we'll want to ask the question of whether we want to do it this way.

My mind immediately goes back to the old Star Wars card games: everyone had a set Power Value and (in the CCG game) Defense Value - they always defended at a certain level and always attacked with a certain amount of force. Less random factor in the game, but it did give a sense of certainty so that Luke Skywalker wasn't being outshone by a stormtrooper (or a naval officer, still more embarrassing.

2) Damage Results: Does damage taken affect your ability to attack/defend? It doesn't have to (in fact, I recently read a post on GMGenie that mentioned people should actually gain stats when injured to reflect rising resolve with rising adrenaline), but if it does, will this change/break the game? We want to make sure we don't suddenly have players running around with 0 Attack or Defense but are still alive, as that makes the game much less fun for that player, :)