If the King is guarded, and you lead the Ace or from the Ace, the King must win; but if you lead from the other hand, there is an even chance that you will find the King on the side you wish. If it is in the other hand, it would probably make anyhow.
Avoid blocking your suit, by leading or playing the high cards from the shorter of the two hands.
As with A K x in one hand and Q x x x x x in the other, play A K x.
As with A Q x in one hand and K x x x x x in the other, play A Q x.
Keep a re-entry card in the hand that has the long suit.
If you are able to take the trick in either hand, do not take it with the hand that has the long suit, unless that suit is established. If you cannot place the lead in the hand with the long suit, it is useless to establish that suit. It is often advisable to refuse to part with the highest card of a long suit, if that card is the only re-entry for the suit.
FINESSING
At "no-trump" the dealer has many opportunities to win tricks with cards that are not the best. In attempting this he should be guided by the following principles.
It is better to finesse on the second round of the suit than on the first.
By forcing discards, you can often tell which adversary is holding and protecting an honour in the suit in question, and on which side the finesse should be taken.