How to use Unity Render Textures with Vuforia?

Hi, I'm having a problem using unity's Render Textures.I have a working scene with an AR Camera (camera1), all works fine here, both in unity player and android devices.

but then I add a second Camera (camera2) which outputs to a Render Texture.. it works just fine in unity player, but when I try it on android device, the background (smartphone's video-feed) is all black, augmentation still works fine .... BUT.... here comes the maddness:

I add a cube to which I applied the Render Texture... what I see there in the cube, instead of viewing camera2's output, I see a crop of the android camera's video feed .. now, I add a third camera (camera3) that also outputs to a new Render Texture, and also apply that new render texture to a new cube... and that texture and cube does work fine, showing the output of camera3.... but why is the first Render Texture intercepting the background video feed instead?can someone give a detailed insight on this issue?as a workaround, I suppose I could use the Vuforia's Background Texture Access sample to restablish the background video feed, but... what should I do with the first render texture that allways fails? create more than one, and don't use the firts one at all? but why does that happen???to be clear:1) the first bug is that video feed on background gets lost..2) the second bug is that the first unity camera to outputs to a render texture, captures a crop of the video feed instead of unity camera's output.

any hints on why it happens, and how to work around it?