Hi,
A possible solution is to link the two cameras to appsink like
v4l2src device=_CAMERA_0_ ! appsink
v4l2src device=_CAMERA_1_ ! appsink
And run a third pipeline starting with appsrc for boradcasting. Here is a sample having one pipeline generating frame and the other two pipelines doing JPEG encoding. It is not same as your use-case but may be a reference:
Starvation (?) of gstreamer threads - #12 by DaneLLL
Please take a look.