Welcome!
my doubt is about using a weapon as an implement and a holy symbol at the same time, weapons like Serpentine Knife of Zannad Accurate dagger or Avandra's Whisper
A few classes can use a weapon as an implement. Warlocks and swordmages, for example. And, if you are proficient in any implement (even a weapon as an implement), you can use it for for any implement attack power that you have, no matter what the classes involved are. So if a swordmage multiclasses and takes some cleric implement attack powers, they can use their heavy blade as an implement to attack with those divine powers.
A monk works differently, and if you look around there will be some threads that explain this in detail (and far better than I can). A monk's implement is a ki-focus, which is basically a slotless magic item. So, if you're using the ki-focus to attack, it doesn't matter what weapons or implements are in your hands. It only matters what the ki-focus does. It's basically an item slot that doesn't have to be wielded.
But it sounds like none of this has any bearing on your situation.
i want to use an accurate dagger an a holy symbol in the same weapon, i have a monk and i take the multiclass cleric feat to take the Holy Symbol Expertise feat
But here's the doubt, when i check my attack bonus i notice that CB is not taking the +1 to attack rolls for holy symbol when i equip this item.. but if i equip an ordinary holy symbol the CB takes the +1 to the attack..
is this ok ?
In theory, this should work. You can use the dagger as a holy symbol, and an accurate dagger adds one to its implement attacks, as long as you are proficient in the accurate dagger. I need more details to explain the CB behavior. What item(s) and power are you looking at? What's the total bonus shown, and what are the character's level and relevant ability scores?
My guess is that the CB is not processing the accurate dagger bonus properly. You have a magic weapon that can be an implement to warlocks (accurate dagger) that has a property that defines it also as an implement for clerics. It maybe checking whether you can wield accurate daggers (and returning a negative value, therefore denying the accurate dagger bonus) before reading that the magic item's properties allow you to wield it as a holy symbol. That's just my guess.
Unless you forgot to take a proficiency in the accurate dagger. That could also explain it.