To manage and deploy your BPEL process in a better way you can create different partitions in the same server. Say if there 3 to 4 developers in your team and you as an administrator want to manage each of their process separately then you can create different partitions in the soa-server for each of the developer and ask them to deploy their processes in their own partition.
As far as question that 2 Managed SOA servers can have 1 MDS schema is concerned it will allow you to create another server it but you may face runtime issues.
If you created a SOA cluster while creating the domain then it will be straight forward as all the objects which are required for SOA Suite will be targetted to the SOA cluster and hence by just adding another managed server in the cluster, you can go ahead. If you did not create a cluster while creating the domain then it will require an expert to re-target all the objects accordingly.