Wis. Stat. § 812.15: Defendant or garnishee may defend both actions.

(1) The defendant may, within 20 days from the service of the garnishee summons and complaint on the defendant, answer the garnishee complaint and defend the garnishment action upon any ground upon which a garnishee might defend, and may participate in the trial of any issue between the plaintiff and garnishee. The garnishee may, at the garnishee’s option, defend the principal action for the defendant, if the latter does not defend.

(2) When any garnishee defends the principal action, the garnishee shall thereby become a party defendant in said action and shall be so entered of record by the clerk, but shall be liable only for the costs in said action.