Hi User123,
Could you explain in a little more detail how the items are related? A relationship implies a 1 to many setup, which I think would cause issues for your use case. The method you're looking at "getRelatedItem" can be used on relationship items only, since relationship items have a source id and related id. If you're trying to get a related item from a non-relationship item, you'd have to specify which of the related item's you want.
If there will only ever be one related item, consider using an item property to create this link. This will allow you to link 1 item of the desired type to the current item, and will even allow you to navigate directly to that item from your item form.
AJ