they work as bi-directional switch( like anti-parallel switches) . however, in ant-parallel switches, if a parallel switch turns on before the previously conducting switch is completely off, it will create a short circuit condition. contrary to this, anti-series connected switch combination avoids this possibility. 

Mr. Shankar, anti-series connection has the emissors of two IGBTs connected together.

Mr. Jagabar, anti-séries connection permit to implement dead time between the turn-on and turn-off of another switch and this bi-directional switch. In this case two gate drive is needed. Besides only one isolated supply to drive is necessary.

Thank you Mr.J.A.Heerdt..

Now i got a answer...so two igbt needed two gate driver circuits..if we drive with single igbt what will happen?