How to filter distinct count

If I understand the problem correctly, you could do a rather brute force and unelegant way.

Have a separate table for your Sales Orders which is all unique values. Relate your Invoices table to your Sales Orders table. In your Sales Orders table, create a column that is essentially (psuedo-code):

Number of Invoices = COUNTROWS(RELATED([Invoices]))

You could also do this as a measure.

Now, create another column that is:

Number of Same Day Invoices = CALCULATE(COUNTROWS(RELATED([Invoices])),[Order Date] = [Invoice Date])

This could also be a measure that instead of repeating the formula just referenced your previous measure.

Create a final column/measure like:

All Same Day = IF([Number of Invoices] = [Number of Same Day Invoices], "Y", "N")

Now you should be able to easily determine how many Sales Orders had all of their associated Invoices post on the same day.