Using two weapons isn't worthless if you don't have the powers, you're just not getting as much out of it. If you don't have the powers, you're getting versatility (ability to use two different weapons based on the situation) and potentially a boost to AC (by using a defensive weapon on one hand, like the cutting wheel or parrying dagger--each of which require a feat, so it's probably not worth it for you). But if you want a completely optimized character, you're better off with something else.
As for optimizing your character...first off, 19 for AC at level 5 seems low. I think the basic rule of thumb for a defender is to be around 17+your level--which in this case is 22. You're a Striker, so the average is going to be a bit lower...I'd say aim for 19-20. As an assassin, you should be wearing light armour, which means your dex mod is added to your AC; by level 5, you should also have a magic enhancement to your armour.
For your ability scores, are the numbers you quote after racial modifiers or before? Dex is your primary stat, so it should be an 18 before mods--at level 5, it should be 21 in total, with a +5 modifier to your attack rolls.
As for feats, I'm not sure why you have four; at level five you should only have three (one each at level one, two and four). Unless your DM has houseruled it. Two weapon defence and two weapon fighting are separate feats, and probably won't do you much good.
On the other hand, if your class features include quick swap and poison use, it sounds like you're playing an Executioner instead of an assassin. (Well, it's a type of assassin, but there you are). If you want to use two weapons, I'd recommend going with Way of the Ninja from dragon 404 as your Guild Attack; it gives you proficiency with the Kusari-Gama, (also from 404), which is a defensive two handed weapon. When you're wielding it, you gain +1 to AC, and your two weapon feats kick in.
One thing to note, though, is that the assassin is built around swapping their weapons around; this is why each of your at will powers uses a different weapon. When you stop using a two handed weapon, you lose all the benefits of your feats. So, for example, if you use your bola takedown at will power, you have to use a bola--a one handed weapon--so you don't gain anything from the two handed weapon feats. With this build, you're better off using those feat slots for something else.
Edit: It looks like an Executioner can have Two Weapon Defence of Sheild Proficiency for free, which explains the extra feat. My bad.