How do I sell inventory to a company that has the same investors/owners?

The transfer should probably be done for fair value. In other words, the transferee entity should pay an arm's-length price (in cash, for a note, etc.) for the inventory, just as a third-party buyer would (and on similar terms---e.g. pricing the goods at market but giving the transferee 10 years to pay, at no interest, is NOT equivalent to an arm's-length deal).

There are a number of reasons for this; whether various reasons actually apply in your scenario is very fact-specific, so I'm just speaking generally.

Creditors are a factor. The creditors of the transferor (if they're smart) will require in their arrangements and agreements that in any such 'sales' of inventory, the transferor must receive fair value.

If the two entities are located in different states, I'll give you 5-to-1 that the pertinent states' tax laws require that the state returns must reflect fair-value transfer prices. Ditto for cross-border transfers.

If either of the companies have profit-sharing or employee bonus arrangements that are keyed to net profits, any inventory transfers that weren't fully priced would unfairly distort the bottom lines of both transferor and transferee (unless a compensating adjustment was made in the bonus calcs).

Of course, if the 'common ownership' to which you refer is a parent-sub (rather than brother-sister) structure and the entities are filing consolidated returns, some of these issues become moot.

Just some random thoughts.