Is it possible to write a piano music using the same G key both for the right hand and the left hand?

I think the types of notation things you are trying to do would best be done in a dedicated notation program. Use one that supports MusicXML and then you can import back into BIAB. MuseScore is free.

I agree with John.

What you want is two staves in treble clef, instead of the grand staff of one treble and one bass clef. Not related to key.

If you get what you want, you will drive pianists crazy.

Yeah Matt is right about driving pianists crazy if this approach is used.

Plus it is not how the staves work. There are too many notes on the piano. It kind of has to be a bass and treble clef. That is why they made it that way.

The piano has a very wide range. You can play everything from middle C on up (or a few notes below middle C) but if you go much below that and stay on the treble clef it will look really weird man.

I get it that sometimes the bottom staff goes to treble, but BIAB isn't going to do it.

Well, that's not quite right. There is no reason not to have a double treble-clef stave. In fact sometimes it's important, critical in fact.
Here is a section from a musical piece I play from time to time, Scott Joplin's Maple Leaf Rag. It would be much more difficult to read that if it was written using a bass and treble clef.

It does take discipline to read though.

Here ... videotrack has understood why I wrote this post. Only for some bars have the treble clef.
To make less effort no more.

jford,
I wrote the musicxml file but now I do not see how to import it into BIAB.

In the last two years of BIAB, it's in the menu Open, Special.

One small thing: you don't have to export Music XML to Musescore. It reads your BIAB .SGU and .MGU files directly. I think it's still the only non-PG Music program that does this.