LOL! Here's how to half-step a Shimano 8:

It can't be done with two chainwheels --- but it can with three.

Example: let's use - arbitrarily - a 700x32 and a 23T cog. Our three chainwheels could then be a 42T, 45T and 47T.

Gear progression would be:
1st ratio, 42/23 = 26"
1st ratio, 47/23 = 29"
2nd ratio, 42/23 = 32"
2nd ratio, 45/23 = 34"
3rd ratio, 42/23 = 37"
3rd ratio, 45/23 = 39.5"
4th ratio, 42/23 = 42"
4th ratio, 45/23 = 45"
5th ratio, 42/23 = 49"
5th ratio, 47/23 = 55"
6th ratio, 42/23 = 60"
6th ratio, 45/23 = 64.5"
7th ratio, 42/23 = 70"
7th ratio, 45/23 = 75"
8th ratio, 42/23 = 79"
8th ratio, 45/23 = 85"

tcs