This is because the dynamic group for trumpet 2 contains the “f” dynamic halfway through bar 4. Remove that “f” dynamic from the group and you’ll get just a single set of dynamics.
Also, why is the passage from m. 24 to the end written on two staves? The two parts are rhythmically identical, as are the dynamics. And indeed on the last system, which continues the same phrase, they are merged.
There are two pitch crosses: the F4 in trumpet 1 in bar 15, and the C5 in trumpet 2 in bar 17. This means that the phrase starting in bar 10 prevents staff sharing. Because Dorico works from left to right, when the next system begins, the condensation setting for the old phrase is still in force, and Dorico doesn’t “know” that the next phrase will allow a shared staff, so it has already decided by that point that it needs two staves.
When condensation changes become available, my expectation would be that if you create a condensation change at the start of the new system, that will be sufficient for Dorico to know that it should reconsider its options from that point, and that will cause it to show a single staff from that point.