Where and what do we have next?

[BoBo]

A ends up short of the line to gain. Say 3rd and 4.

USC like on B first and then USC on A.

We march of the 15 yards on B first. This now results in a 1st and 10 for A.

Then we march 15 yards off on A and it is still 1st and 10 provided no half the distance walkoff on B first.

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So on the above play my interpretation is correct or not.

Just a simple yes or no