Dear Cindy
I have to accept your answer; but I have never heard about one "group of companies" not sharing master data. So your scenario seems to be this:
a.) you would like to use one SAP ERP for all companies
b.) per client one or more companies will act
c.) in the SAP ERP you have prepared EHS/EHSM
So the answer is simply: yes you can use thise scenario but as with SAP ERP SAP EHS does have the need to prepare client specific set up. There only to give you some ideas why this is a "bad" choice some list of issues:
a.) per client you have to prepare EHS customzing
b.) per client you need to load data (e.g. if you use an external data provider)
c.) per cllient you need to prepare suitabale user roles (access concept)
d.) per client you need to establish e.g. MSDS/SDS distribution etc. etc.
e.) per clientyou need to prepare WWI server set up as well as Expert server (if needed)
f.) etc. etc. etc.
Depending on the number of clients this is a "bad" choice/option. to make it quite clear:
"the backend ERP could source the master data accordingly?"
This does mean: you have client 100 (as example) there company A is acting (you maintain in client 100 master data). And then you as well establish EHS/EHSM in same client; then the same for client 200 etc. BUt per client you will need "similar" customizing in SAP ERP as well.
Personally I can not believe that there is no overlap in vendor/customer etc between the different company which act. But you need to decide. Technically: you can install a lot of clients in the same system; but e.g.you need to replicate your access concept per client (as you need the "same" SAP MM/SD etc. process in most cases by client). Depending on the number of clients you would like to install/prepare this can get a "disaster".
C.B.
PS: not sure about your remark: The only similiarity would be the configuration of EHSM which would be client independent and the same for all companies.
This is technically not possible. Per client you have one EHS/EHSM customizing. If you have two clients you need to execute the EHS customizing twice.