well it turns out this is not a great idea, I heard back from Bowker:

Splitting a block of ISBNs at all is absolutely not possible. All of the
ISBNs in a block are identified by the same prefix—the first several
digits of all the numbers in the block are the same. And this first
several digits—the prefix—can only identify one company or person. It
is not possible to have a block of ISBNs identify more than one person
or company.

When people try this they only find out that bookstores will not take
their books because they do not own the ISBN (they are not the publisher
of record).

So please do not do this. It will only cause confusion in the
marketplace, the inability to register titles in industry databases,
and, of course, authors who were once friends quarreling amongst
themselves.