I have performed the same mod, but using only precision cutters.
It's nice because the batteries sit side by side.
I break off the tabs that touch the original battery, leaving the charging contacts and wires to the PCB. I then solder my battery wires to the PCB. where those wires meet up.
I did use a dremmel to open up the PCB cover so I could re-use it.
Here is a link to my
Yahoo Photo Page showing the mod.
Once you know where to cut and prepare your dual batts for installation, the mod goes very quickly.