How would that work?
You'd have to make the entire case wider for a start.
There's a lot of internal framing that'd get in the way.
If you're willing to work at it hard enough it may be possible, but there wouldn't be much of the original chassis left.
Why do you want two slots anyway?
Which dual slot GPU are you going to install in there that'd:-
There must be a more modern case that'd suit you better.
- Not cook due to the limited space and airflow?
- Run on the low capacity PSUs you can fit in there?
Jesus H, that thing is massive. There are plenty of dual-slot ITX cases out there which are smaller.