Thanks Steve for your reply,
Actually we have 300 Building, half them connected already by ISP1 (70Mbps) and another building we are planning to connect it to the new ISP(1Gbps).
Scenario 01, I need to know how to keep and use both ISP in some fashion as you mentioned.
Scenario 02, I need to know how to make it (ISP2) for the remaining building where we want to activate the Internet.
Regarding your questions:
Do you have inbound destination nat traffic for internal services? yes which connected already to the existing ISP1