How can I create a Formula field that grabs a value from another object?

In this case I have two custom objects: "Sales Order" and "Sales Order Item" 

In the formula editor my only choices are the Object that I am working in (Sales Order) and other objects like:

$System

$User

etc.

If this makes sense...I want to create fields in the "Sales Order Object" that derive their value from the "Sales Order Item" object.

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:smileymad:

This is actually part of a bigger problem that I am trying to solve with Email Templates that won't allow me to get Merge fields from more than one object in the same template.

i.e. I create an email template that uses my Sales Order object...  the interface lets me select and insert merge fields from other objects both Standard and Custom... but those merge fields don't actually show up when I see the email

Any thought on either would be greatly appreciated