This, in my opinion, is the key question.  If the tables are related, I'd create an identity column on the parent and then the child table could reference the parent table by way of a foreign key.

If the tables aren't related, then they wouldn't necessarily (from a database perspective) need to have sequential numbering across both of them.  If there's a business need that they have unique numbers across both of them, you might want to look into a sequence object. See https://docs.microsoft.com/en-us/sql/t-sql/statements/create-sequence-transact-sql. Just be sure to start with the minimum value you can accept when you create it.  There's a section on that page on caching you'll want to understand as well.  This might do what you need.

I've not had to use sequences in SQL Server, but I have in Oracle years ago because Oracle didn't have an identity property for a column back then.  I don't know about now.