Yes, the conventional concrete was cast first in a trial girder that was manufactured for FDOT. The interface is the primary research item, and it was found that the bond across the joint between materials was very good and could not be made to fail. A summary report and final report are available on the testing (see https://www.fdot.gov/research/documents.shtm):
BDV31-977-101 Hybrid Prestressed Concrete Bridge Girders using Ultra-High Performance Concrete Summary [245KB] Final Report [15.6MB]
The bond of UHPC to a roughened and prewetted (SSD) existing concrete surface is well documented. The strand and/or mild reinforcement that crosses the interface also contributes to the behavior of the hybrid product.