Making the leap from siloed organizations, measured independently and using disparate solutions, to a single process based on an intelligent demand signal is going to take some effort.
The first step is defining the end-to-end planning, allocation, and replenishment process and determining the common metrics on which to measure the process. This will depend on the current state of your systems and the visibility and information available to your planners, allocators, and replenishment analysts as they perform their work. This will also depend on the flexibility to incorporate a demand forecast into the allocation and replenishment execution.
From an organizational perspective, this change will require buy-in from both the buying/merchandising and supply chain executives in the company. With strong collaboration, both benefit in the short term. The buying executive gets better visibility to demand and maintains higher service levels, while the supply chain executive reduces overall inventory in the network and carrying costs.