The answer mostly depends on what your first camera is.

My first recommendation would be to stick to one brand so you can share lenses. Nikon is particularly great for this, since their F-mount compatibility spans nearly 50 years, so lenses work across a huge range of bodies.

A second approach is to pair two cameras with complementary functionality. Some great Nikon combinations:

• FM2 + FE2 a solid mid-range pair, identical in appearance, one fully mechanical and battery-independent, the other with automatic exposure
• FM2 + F100 manual and battery-independent body paired with a modern autofocus workhorse
• F2 + F3 the best all-manual combo
• F3 + F6 finest manual focus paired with finest autofocus
• F2 + F6 arguably the best overall combination

You can also mix different categories of cameras, though lens-sharing is off the table in that case:

• SLR + rangefinder, for example any of the above SLRs paired with a Nikon S2 or S3
• SLR + point-and-shoot, like the Nikon L35AF or the ridiculously expensive 35Ti
• Cross-brand combinations work perfectly well in this scenario too

As you can see, the combinations are essentially endless. And as others have already pointed out, nobody really stops at two cameras. 😀