Updated to Catalyst 9.2 - the problem still exists!
AntiAliasing is inactive after the first draw call to multiple Rendertargets. Using the same setting, NVIDIA-Cards are NOT affected.
Updated to Catalyst 9.3 the Problem still exists...
This may be a bigger Problem, since it will possibly effect a range of Games, failing any form of AntiAliasing, as soon as multiple RenderTargets are used in any stage (e.g. Shadow Mapping or Post Processing)