AssetBundles and Vuforia to load 3D models

Hi,
I'm having problems making an AR app using Vuforia and AssetBundles.
I want load my 3D models from an external server (dropbox in my case). I follow the steps of this link: https://developer.vuforia.com/forum/faq/unity-how-can-i-augment-my-image-target-model
Everything works fine when I add the script include in the link to my "image target". The problem is when I create a second "image target" and I also add to this target the script with another url that downloads a second AssetBundle from dropbox. In this case the 3D model appears before scaning image target and disappears when the image target is tracked. The first image target still working fine but not the second one, that disappears when is tracked and not appears again despite of the target is found.
What I have to do to set multiple image targets that call 3D models from server and these models only appear when their image targets be tracked?
Thanks