Mounting Bearing Blocks

Think Archemides. And stability at higher speeds.

You can calculate the load at each bearing using the leverages involved. The less the spread between the two bearings the higher the load on the bearing near the free end. (This bearing load will always be higher than the load at the opposite end.)

If you do a few sketches, and calculate loads, you'll see that the load on the bearing at the overhung end goes up as the bearing spread decreases. When the overhang is equal to the bearing spread, then the overhung side bearing has twice the load of the opposite bearing. The stress on the shaft will be highest at the bearing on the overhung end, and this bending load and deflection is reduced by reducing the overhang. The shaft stress is cyclic (in other words, the shaft bends first one way, and then 180 degrees later in rotation bends the other way) so there is the potential for fatigue failure.

Therefore, as a general rule, you should keep the overhang to a minimum, for both bearing load and shaft flex/failure reasons. This Gates paper addresses some concerns to be considered for belt drives in which the additional tension required by a belt (vs a chain drive) may over-stress the gearbox or shaft.

the distance between the two bearing blocks should be twice the distance between the second block and the free end

I'd edit that to read: "blocks should be at least twice the distance..."