Is it existing a kind of tripod head that can carry two apparels in the same time in parallel (or one over the other)?
The idea (I do not know if it is recommended or not) is to have for example a camera on one side with e.g a 300mm lens, and a fieldscope on the other side.
The goal is for watching birds and potentially make pictures (if they come nearer) and use only one sturdy tripod instead carrying two tripods or switching between camera and fieldscope.
Yes there is. It is used for 3D photography. There are probably other models but that is the model I know. Not sure what exactly generic name should be but I would look for more double camera or dual camera support.
Just to be clear. You attach this to your tripod head and mount both cameras on it. You there is no to get another tripod or head to use this.
There are a number of ways you could do this. You are really looking for what is known as rigging. You would mount the rigging on the tripod body and use heads that can attach to the rigging. B&H or any other professional A/V store should have a variety of rigging available, though it may be cheaper to make your own as rigging tends to be pretty pricey since it is specialized devices.
