Sequential numbering service program +ILE architecture

I have two sets of programs/files that can generate invoices, one for
parts and one for tooling. They exist as two separate systems. It
appears that one of the reasons for making them separate is that they
could both be run at the same time by different users. The way the
invoice programs are written, if both systems used the same invoice
numbering storage, duplicate numbers could result.

I wish to more closely integrate these separate systems, however to do
so I need an invoice numbering routine that would ensure that no matter
how many invoicing programs were in use each program cycle would get the
next unique sequential number and not cause record locks.

It seems that a service program would be in order. The question is, how
do I make sure only one instance of the procedure can run?

Or, maybe I'm asking the wrong question. How can I make sure to
programs that run simultaneously can be guaranteed a unique sequential
number?