You can also take metro from Ataturk Airport to Yenikapi. You will change to line called Marmaray from Yenikapi to Ayrilik Cesmesi and than you will change the line again to get Metro which goes to Kartal. After you arrive to Kartal District, you can take Public Minibus or Yellow Taksi to the Sabiha Gokcen Airport.
I can not say it is easy to travel with subways from Atatürk Airport to Sabiha Gökçen Airport.
You can also take metro from Ataturk Airport to Yenikapi. You will change to line called Marmaray from Yenikapi to Ayrilik Cesmesi and than you will change the line again to get Metro which goes to Kartal. After you arrive to Kartal District, you can take Public Minibus or Yellow Taksi to the Sabiha Gokcen Airport.
I can not say it is easy to travel with subways from Atatürk Airport to Sabiha Gökçen Airport.