DAX Measure - Co-occurrences

You'd need to create a separate table for use in the slicer. You can use

Product for slicer = DISTINCT( 'Table'[Product] )

Do not connect that to the main table, just use it in the slicer.

You can then create a measure like

[Num co-occurences] = IF(
SELECTEDVALUE('Product for slicer'[Product]) <> SELECTEDVALUE('Table'[Product]),
VAR InvoicesForChosenProduct = CALCULATETABLE(
	VALUES('Table'[Invoice no.]),
	TREATAS(
		VALUES('Product for slicer'[Product]),
		'Table'[Product]
	)
)
VAR InvoicesForCurrentProduct = VALUES('Table'[Invoice no.])
VAR Result = COUNTROWS(INTERSECT(
	InvoicesForChosenProduct,
	InvoicesForCurrentProduct
))
RETURN
	Result
)

and use the [Product] column from the main table in your visual.