This is kinda a starting point for an Assassin/Illusionist that I would use:
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It's good to have the lover's stone for this kind of build since you will be using skills under all three signs. This is just kinda something to give you ideas. You can invest in archery or destruction if you want for ranged damage, but remember you can get sneak bonuses for archery. It's also a good point to bring up that Vamprisim, while sometimes challenging can benefit this kind of build a lot! Sneaking and illusion magic will both greatly benefit from being a vampire.
Now, more about specific skills... I don't dual weild daggers, but you can (it's unweildy and seems over powered for me). But dual weilding works for some... it just seems unnecessary for this build to me, since you can one hit kill a lot of things with a backstab and assassin's blade perk anyway. You can go all the way into the sneak tree, but some people find the light foot, silent roll and the other two perks on that side a little pointless once you get that good at sneaking. You don't really need too much in light armor, since idealy you wont be getting in to direct confrontation too much.
Now about illusion: dual casting illusion spells is important. This will allow you to affect higher level enemies with your spells. Illusion spells can be your friend a lot. You can calm enemies if you are in danger or make them run away. Obviously muffle and invisibility help sneaking a lot. There is nothing more fun for me than using fury to make enemies kill eachother also. Silent casting is important for this build too!
One last thing, you may or may not want to invest in the master level illusion spells. Dual casting the expert level spells will allow you to affect higher level foes since you cant dual cast master level ones. The only thing that master level spells give you is greater AOE.
Any questions?