To bump up the backstab multiplier, the easiest way would be to download Near Infinity here:
http://www.idi.ntnu.no/~joh/ni/
To install it you will need Java Runtime Environment here:
http://java.sun.com/j2se/1.4.1/download.html
Download the JRE option, not the SDK option.
Boot up Near Infinity
There will be an explandable tree of different files types on the left hand side of the screen.
I believe the very first file type is .2DA
Click on the tiny button for .2DA
The list will expand.
Scroll down until you find BACKSTAB.2DA
Click on it.
You'll see what looks like a text file - it basically is.
It might look something like this...
2DAV1.0
1 1 2 3 4 5 6 and so on
Thief x1 x2 x2 x2 x3 x3 and so on
The top row is the level, the bottom row is the multiplier.
Right click on the BACKSTAB.2DA file, you will see an option for export.
Export it to your override folder.
Go into the override - open the .2DA with a text editor - adjust the multipliers as you see fit, and then save it.
That should do it.
As for making two-handed swords usable for backstabbing - I believe Bardan is correct. You can make individual items "flagged" for backstabbing - Near Infinity or IEEP should work for this.