There is no prohibition on owning more than one short upper if you own only one lower. There is, however, no law prohibiting owning more than one shorty if you have a lower that is non-SBR either. The laws, however, can be interpreted to read that an extra shorty upper in the presence of a non-SBR lower constitutes 'constructive posession' of an illegal SBR. The ATF has issued a ruling to this effect. While not 'law' per-se, they are the ones that do the enforcing, and it is their decision.
