When was this made illegal?
@Peter Grove
This is a slightly tricky one and it somewhat depends how you look at it.
@ARF has summed up entirely correctly the situation in terms of what is written in the law (nothing mentioned about control before 1991 and the stuff related to preventing the keeper releasing the ball, not until 1997).
So prior to 1997 it would be difficult to say whether this was prohibited or not since there was nothing specific in the law even vaguely related to it (the clause about control added in 1991 was to do with the four-step rule and nothing else) and even after 1997 it could be argued whether this was actually preventing the keeper from releasing the ball or not - though I would rather tend to think that it would be seen as a prevention of the keeper's release of the ball.
The "tricky" (and slightly confusing) part is that the FIFA Q&A's in 2004 had the following:
However, the very next year this was reversed:
I suspect this may have been because they realised it would just open a whole can of worms where players would be trying to knock the ball out of the keeper's hands by whatever means, or perhaps they thought that heading the ball out of the keeper's hands went against the provision about preventing the keeper from releasing the ball.
But we'll never know because they never explained why the change was made (or rather, why they ever said it was allowed in the first place).
In a reply to a tweet he says that the laws stated that a goalkeeper only has control of the ball when he has it in both hands
There has never been mention in the law of both hands being required for control. As mentioned above, there was no mention of control before 1991 and ever since that time it has only said "any part of his/the hands or arms" which has always been typically interpreted as meaning just one hand is required, to the best of my knowledge.