This is possible because the two metaphors for time pick out different aspects of the target domain. The coming weeks conceptualizes those weeks as a whole, in motion relative to the observer. Within looks inside that whole, conceptualizing it as a bounded region with an interior. Each mapping is used partially. The Invariance Principle allows such such parts of the mappings to be picked out and used to characterize reasoning about different aspects of the target domain.
Simultaneous mappings are very common in poetry.
Take, for example the Dylan Thomas line Do not go gentle into that good night. This one line has three different, metaphors for death, each mapped onto different parts of the sentence. This is possible since mappings are fixed correspondences. There is an important lesson to be learned from this example. In mathematics, mappings are static correspondences.