Two loading bays are connected together, share the same name. How to let the trunk use each of the (random) two loading bay? Now it only can use one.
Simutrans doesn't work with random allocation of platforms. Vehicles go to the tile you order them to. If that is occupied they wait until it is free. If there is a choose sign at the entrance of a station they will try to find another platform if possible (just be careful to put an end choose sign at the exit of the station if you do this).
There is an image at http://forum.simutrans.com/index.php?topic=7871.0 (http://forum.simutrans.com/index.php?topic=7871.0) showing the use of a choose sign, but note the comments in the following post as to what has been done wrong in that image.
Note also that choose signs, and choose signals, can end up doing things you don't want if you're not careful. The sign(al) must be placed so that only the platforms you want passing vehicles to choose from is behind it, and don't put the sign(al) along the path of any other vehicles. An end of choose sign(al) can somewhat counter the former requirement, but not all pak sets have end of choose signs.
Is the signal named: change destination sign?
It depends on the pakset - pak64 calls them "choose points" or for railways, "choose signals" and then "exclude from platform choice", pak128.Britain "choose sign" or for railways, "platform choose signals" and then "end of choose signal". For roads you don't seem to have end of choose signals generally.