With two good batsmen batting, it doesn't matter much who faces any particular ball, so they will score runs whenever they can and swap ends when they happen to score 1 or 3 runs. They tend to like swapping ends, rather than remaining at one end for an extended time, since it means the bowlers need to adjust to bowling to a different batsmen every few balls. This can be especially true if one batsman is right-handed and the other left-handed, since the bowling line to attack each batsman will be different.
The situation changes when enough wickets fall that a relatively poor batsman comes in to bat with the remaining good batsman. In this situation, it is tactically better if the better batsman faces most of the balls. To do this, the better batsman will tend not to take 1 or 3 runs even if they are available, restricting himself to 2s or boundaries, during the first half of the over. Depending how confident the batsman is that his less skilful partner can survive a few balls without getting out, the better batsmen will try to score 1 or 3 on the 4th, 5th, or 6th ball of the over, thus giving him the strike again at the start of the next over. A batsman who can score 1 or 3 off the last ball of the over consistently can face several overs in a row without the poorer batsmen facing any balls at all. This tactic is known as farming the strike
