Iām guessing it would be easiest to do this with the Line tool, in Write mode.
That will work, but Dorico has made provision for this by doing it the following way:
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Enter an independent time signature for that staff - select a note on that staff and confirm it using alt-Enter (instead of just Enter). Hide the time signature (in Properties).
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Enter the barline at the point where you want it to appear (in case extra beats are added, you might need to enter it as a double barline and change it to a normal barline).
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Select the note on that staff immediately after where you want the barline to be changed to a dotted barline then open the Barline popover, type Dashed, then confirm it using alt-Enter.
You can do that with independent time signatures for those bars on the bottom staff.
Select bar2 of the bottom staff. Do shift-M 1/4|2/4 alt-enter. Reset it in the following bar. Hide using the properties.