Sharing Calendars and Processing Meeting invitations between Notes and Exchange

We have changes with managing calendars and resources between 2 organisations who operate a substantial joint venture

Organisation 1 is Lotus Notes, 400 users

Organisation 2 is Exchange 6000 users.

Org 1 and Org 2 create a joint venture which is 60% of org 1 and 5% of org 2 by revenue value.

Joint Venture has single management structure across the 2 organisations.

Ability to collaborate is important.

Issues:

Problems with processing meeting updates (orginal meetings work fine). Issue is when invite is initiated within Exchange, Notes is unable to update changes to the meeting. Seems to work ok if Notes is the initiator.

Inability to share freetime and resource information. Need to be able to organise meetings including booking rooms and Audio or Video sessions.

Any ideas appreciated.