In the World Pool-Billiard Association rules of 8-Ball, you only call one ball and one pocket on each shot. As long as that ball falls into that pocket, your turn continues. If it does not, your turn ends and your opponent takes over. Any balls that fall into pockets on your shot stay in the pockets.
If you intend to pocket two balls in one shot, you only need to call one for your turn to continue.
Keep in mind that there are different rules used for different tournaments.
From WPA Rules of Play, General Rules, 1.6 Standard Call Shot:
1.6 Standard Call Shot
In games in which the shooter is required to call shots, the intended ball and pocket must be indicated for each shot if they are not obvious. Details of the shot, such as cushions struck or other balls contacted or pocketed are irrelevant. Only one ball may be called on each shot.
For a called shot to count, the referee must be satisfied that the intended shot was made, so if there is any...