In many publisher submission guidelines,
they state
that they won’t accept manuscripts
submitted for consideration elsewhere.
That is a lie.
Because they accept (and prefer)
manuscripts from agents.
Agents submit to ALL publishers
at the same time.
They do that with a hope of creating bidding wars.
There are no rules in business.
There are laws
which you should follow
if you want to stay out of court
(and jail).
There are guidelines
(ethical and other)
that may or may not help you
be successful.
But there are no rules.
For every rule,
there is someone breaking that rule
and becoming VERY successful
by doing exactly that.
Be conscious of your ‘rules’
and ask yourself
‘are they limiting me?’