Since Access security is "roll your own" or "none" (on a native solution), and you WILL want to keep things separate as a matter of fiduciary responsibility, I'd say that separate BE files would be the way to go. First, it is FAR easier to assure that users cannot overlap if you can use Windows domain-level security as a factor in that solution.
However, another factor pops up in my mind. If you are going to have multiple users from multiple companies on a single BE, you have VASTLY expanded your exposure to the random true hacker or hacker "wanna-be" types that are in every company.
There are the "curious George" types who will browse anything that isn't nailed down. There are the "grumpy Gus" types who will SHOW you that you can't keep THEM out of a system. There are the "helpful Harry" types who will tell you "Your security would be better if...." followed "... because I just found THIS on the alternate paths." Not to mention "Carmen the corporate spy" who will work assiduously to find some tidbit of saleable corporate "insider" knowledge.
If you can keep them physically separate because they don't have rights to anyone else's folders all the way up and down the path from the BE server to their particular database, you are better off. And this is what firewalls are meant to do. With THAT kind of security and physical separation of the sub-nets, your security is not going to be so much of an issue. Besides which, most customers want to believe that they have a dedicated system all to themselves.
If you are hell-bent on a single-BE solution, just remember that some products have size limits. If you have something that can go up to 10 GB databases, just remember that if you have as few as six customers, you just crossed the line in BE size. Because with "pure" Access and a split FE/BE you can have 2 GB of data, but with a unified BE and a 10 GB limit, six customers only have an average of 1.666 GB each.
Not only that: You will have to arrange for maintenance times and with five or six different corporations, you will have PURE HELL to get them to agree on a regular schedule of down times.
Here's a thought: If you are going into this in a serious way, look into something like VMWare or some other provider of virtual host management. Their cross-client security is excellent and with some network-attached storage to bolster available data space, you can have a pretty good-sized server farm. Yes, NAS is quite capable of partitioning itself to keep multiple projects and data separate.
My last job before I retired was with the U.S. Navy's Enterprise Data Center in New Orleans. At the NEDC NO site, we had a few dozen VMWare systems hosting several HUNDRED virtual systems spread out to service over 80 different Navy and other government groups. We also had TERABYTES worth of NAS. Not only that, but we had systems that maintained SECRET-level security. If you think about the stringent security needs of the USA armed forces, you would realize that our setup was under constant scrutiny for holes, leaks, and breaches. But we WERE considered a secure site. And we found that keeping projects on separate sub-nets and servers from the moment they came online was the best way to keep their data separate AND SAFE.
