Hi Borut,
I would like to answer your questions one by one.
1. Can I use the same ADFS Farm to add trust for south.com users to different Office 365 tenant /subscription.
A: Based on the customer’s feedback, it is feasible to do it. However, there are no official articles regarding it.
2. Can I use same Haybrid Exchange server for both Office 365 tenant / subscription.
A: No. For a single Exchange Orgnization, you can only connect to one tenant or the other via Hybrid wizard. You cannot split one Exchange organization amongst two tenants.
Thanks,
Johnny Zhang