The judge
It depends on the sentence for the second offense. If the judge sentences you to serve the second sentence consecutive (after) to the first, you will serve the remainder of the first sentence before beginning the second. If the judge orders the sentence concurrent, you will service both at the same time.
The criminal was sentenced to two jail terms to be served concurrently.
Yes, you can have the sentence changed by the judge that gave it to you. However, it's not free. You usually have to give evidence against yourself or someone else, to get the judge to want to change your sentence.
There was thunderous applause when the judge sentenced the man to 67 consecutive life sentences.
Taking the information contained in the question and doing the math - it sounds like a total of 9 months backup time. If the PO recommended that 3 months of that run concurrently - then that would mean the jail time would be only 6 months.
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The judge
A "bench trial" is a trial before a judge sitting without a jury. The judge alone decides the case.
In the US, the arraigning judge decides typically whether bail will be assessed, and how much. The judge at the preliminary hearing may modify this, or any other judge to whom a motion is submitted.