You can only work 20 hours per week, so if you have a GRA appointment, you will need to reduce your hours to 10 in order to accept a TA position.
You can have a 25% TA appointment in another department to reach 50%, but you will need to apply for the position you are interested in. If you have a 50% appointment that becomes a 25% appointment, your appointment will be rescinded and offered to a candidate who has a 50% appointment.