The home leg will be on Friday 14th May, and the away leg on Monday 17th May both kicking off at 7.45pm.
Big queues formed at the County Ground as tickets went on sale on Sunday.
On Sunday and Monday tickets are on sale to season ticket holders.
On Tuesday fans with a ticket stub from the Brentford game wil be able to buy a ticket, and from Wednesday the rest will be on general sale for the home leg.
Away tickets are currenly on on sale to new and existing season ticket holders, but may go on general sale if there are any left over.