Distinct count filtered where distinct count=1

I have two tables, one with orders, and one with the items on the order.  They are linked by a separate key:

In the example I have:

• 16 rows of data
• 7 unique items
• 14 unique orders
• 5 unique orders where there is only 1 item

I am looking for a measure that will give me that last output. i.e. a distinct count of the order, where the distinct count of the item = 1

I tried below but it is gving me a zero output

CALCULATE( DISTINCTCOUNTNOBLANK(Table1[Order]) , 
FILTER( RELATEDTABLE(Table2), DISTINCTCOUNTNOBLANK(Table2[Item])=1) )