وَلِتَبَادُرِ الْفَهْمِ عِنْدَ الْأَمْرِ بِشَيْءٍ بَعْدَ الْأَمْرِ الْأَوَّلِ
And because a second command following the first prompts a particular interpretation.
A second order issued after a first can change how the listener understands the timing or sequence of the two requested acts. If the acts cannot be combined in the relevant manner, the listener may infer that the initial command anticipated prompt execution or that the later instruction alters the earlier practical situation.
This is an argument from immediate pragmatic understanding, not a universal claim that any two imperatives are inherently incompatible. Feasibility, duration and the possibility of fulfilling both have to be assessed.