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Either side can call the other side as a witness. The difference is that when a party calls a witness for direct examination, the party cannot lead the witness - meaning the party cannot ask questions that suggest the answer or a question that may be answered by a mere yes or no. Leading questions are usually only for cross examination of the other party's witnesses.
An exception to this non-leading questions on direct rule is - when you call the opposite party on direct (not their witnesses but the party his/her/itself) you can ask the judge that you treat them as a "hostile witness" and be able to ask leading questions.
So to answer your question directly - yes both the plaintiff and defendant can call each other as witnesses. And both can treat each other as a hostile witness on direct examination, which allows them to ask leading questions to each other.
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