Hi Lalit,
Do you mean some reference about this configuration?
Integrating the multiple-forests environment with Office 365 online AD is not achieved by the normal DirSync tool. We should use FIM Connector. Please find more information referring to the link below:
However, at present, there is not an officially release about the detailed deployment. I'm so sorry that I could not provide the detailed instructions from my side. Your understanding is pretty appreciated, Therefore, I suggest you check if you can get any specialized support from Microsoft partner.
Thanks,
Young