Because wireless dog fences are created by radio waves sent from a central transmitter unit, these waves radiate in a circle.
The circle can be made smaller or larger, but it can't change shape.
Combining two or more transmitters will allow you to cover your yard more thoroughly, though the shape will always have a rough, scalloped edge.
This may be enough customization for some, but those that have extra space or odd areas of land attached to theirs might need more of a customized boundary line like a wired dog fence.