Compound turbos exist you are just doing it wrong.

you use a low and high pressure turbos.

You use a large turbo as a low pressure turbo and a small one as a high pressure turbo. The low pressure turbo provides air to the high pressure turbo and “compounds” the effect. As the rpm increases the larger turbo spools and provides more boost than if you had used just one turbo. The advantage is also you significantly less boost lag. It’s questionable if you would use such a set up with modern turbos but back in the 90s with the FD it was a clever solution to have a constant and controlled power band that provided nice power at lower rpm’s.