I have set up and worked on 4 multi company groups with Single logistics and Multi finance and a mix of multi Logistics in some instances.
As you probably know by now the enterprise unit set up is a major issue and the DEM modeller set up must be correctl to ensure correct vissibility and financial posting.
The biggest issue will be setting up your shared tables to ensure that you get the cross company vissibility required. Note that sometimes it is easier to decide what the business issues are and to then research wether it is a single logistics compny set up you require, or should you just share warehousing tables for example to see inventory availability. Note that in my experience single logistics requires clearly defined business rules and discipline. Table sharing rather than true single logistics can sometimes provide a better solution.
Other solutions I have set up have involved a shared logistics company where a group of companies are set up but only agreed shared data is vissible in specific warehouses, purchase and sales offices, this can also give more control over who is selling and moving what.
You cab email me at firstname.lastname@example.org
with more specifics.