When building an anchor for rope soloing, specifically a natural anchor (chocks, cams, nuts, hexs whatever) you have a lot of freedom. The anchor has to, in general, be able to hold a rappel or downward pull as you second the pitch but it also has to be able to hold an upward pull as you climb the pitch above. Opposing pieces can be very effective however if they are directional it is important to make sure they are not pulled in the wrong direction. Pulling a chock backwards will most likely take it out of the rock and then you could get hurt.
To ensure that your anchor is stable in the direction it is pulled use clove hitches to keep tension between two or more pieces, use load limiting runners to reduce the force on a piece or in a direction of pull, use a haul bag as a damping weight against a leader fall, or use multidirectional pieces such as bolts or natural gear (cams mostly) in horizontal cracks.