Hi,
If you visit flowersbyfrederique.com, and just put anything in your basket, you'll see the checkout with the two delivery date fields.
In the video, I demo the behaviour. First at flowersbyfrederique.com then at verbloemen.nl
At verbloemen.nl I run the exact same DeliveryDate extension, but with the GCK store template from Shape5.
Video: https://www.screencast.com/t/X7UPlFm8DrGf
What login info do you need?