1. I'm pretty sure that Venus doesn't have an appreciable magnetic field.
2. Even if it did, its day is about the same length in its year (e.g. about 250 earth days) so nobody could live in any fixed place on the planet without freezing or melting, even if we got rid of the thick atmosphere. You'd have to live in trucks rolling slowly around the planet in the
Mars on the other hand has normal days and could be warmed up with a greenhouse effect. Also, the thicker atmosphere would provide additional sheilding at the surface level. One could imagine the last few percentage points of shielding being made up with local magnetic field "bubbles" around settled areas, powered by fusion reactors, assuming we have that technology in the next century or so.
The test of intelligent tinkering is to save all the parts. -- Aldo Leopold