If the visitor medical insurance plan lists both of you on the same plan, you cannot cancel coverage for only 1 person and continue coverage for the other. If you have separate plans you can email your cancellation request for the person who needs to cancel.
If both of you are listed on the same plan you may also consider cancelling your current plan and purchasing another visitor medical plan to cover you alone. Since yours will be a brand new plan your deductibles, co pay and pre existing condition waiting period will restart from the day your new plan begins. For the maximum flexibility purchase separate plans the next time you need visitor medical insurance.