If you are playing with two tables of ten, you need a maximum of four movements before you collapse the tables into one:
you have to lose a pair of players from one table only, in order to justify a single movement
the fourth pair causes the fourth movement, but the fifth pair causes the collapse (because five pairs makes ten players)
Since this is extremely unlikely without some more interesting situations to worry about, you have no reason not to maintain balance wherever possible, and should do it as soon as possible to prevent the negative effects of imbalanced tables.
However, as three or four players is 15 or 20% of your player pool, you should aim to avoid unfairness by requiring one player to move more than once, and by moving them to positions with different advantage.
This leads to three simple rules.
As soon as there is a difference of more than one player, make a movement (since this is the earliest chance to achieve actual balance).
Move the closest eligible player ahead of the blinds to the big blind on the smaller table. This avoids the disadvantage of moving from good position to poor as much as possible, and avoids the lack of knowledge of players on the new table from having much larger detrimental effect relative to the ability to use that knowledge effectively (viz. a lack of knowledge makes a good position much worse, while it doesn't really change much for a bad position)
Never move the same player more than once in a tournament (since this prevents the disruption affecting one player disproportionately, while also preventing them from obtaining knowledge about players on both tables prior to the collapse, and having a disproportionate advantage from their movements).