This is the way I do these:
- install the free ArcGIS Explorer program
- download shape files for US Counties and US Congressional Districts from the US Census Bureau
- load both shape files as layers.
- change projection to Mercator (World Projections (Ellipsoid))
- change background of both layers to be transparent
- change county layer to be in front with black boundaries
- change CD layer to have a different color boundary
- scale the map until it matches county dimensions of my individual state county maps. (this takes me a couple of iterations to get it right).
- screenshot the map
- use an image manipulation program's lasso tool to surround/select a county split boundary. Copy and paste into the county map.
- then clean up with pencil tool/eraser.
- repeat until all county splits are complete.
Some of these can be quite congested - such as Harris County TX -> at this scale, its tough to do. I've attached an example.