Can't say anything before checking your workbook. Can you share the sample workbook ?
The stretch problem comes from missing value in a date dimension as you certainly know.
To get around this, If you have a date dimension that is alway filled (or mainly filled), you can make it as your "Master Date".
For the other dimension in which you have no date (and no data to display), take the "Master date" value with a calculated filed:
IF date2 is null then [Master Date] end
Display your date with your Master Date field on the first Viz and with your new calculated field on the second Viz like you already did.
(We cannot see your dashboard due to connexion request. You may have to do an extract)