In the circumstances explained, it's implied that you are also willing to join Co.A in due course. Which gives an impression that job in offer in co.B may not be of your choice. That apart, It's possible to info Co.A that you they may stop further process in your joining formalities. You are free to write a convincing mail to them expressing your inability for the present. At the same time keep the option open for the future. But, to my knowledge, many employers may not like to entertain profile of a candidate who declined their offer once. Ofcourse, except in exceptionally promising candidates whose potential, in their judgment is good for their Co. Many employer like to go for 'head hunt' from their competitors wherever they are eager to grab businesses/clients of revels.