But start both now. Ensure that your steering committee controls both projects. You have less than 10 months. You must plan at least one month for testing. The essence is that both projects have the same schedule date.
Time is very short.
It is crucial to have at least started on the Inventory side of Y2k within two months. Keep this phase short. I have heard people talking about 3 months for Inventory. We cannot spare that time. Maximum One Month.