Hopefully, the installation was engineered and commissioned to accommodate phasing and synchronization of the two sources, whatever they are. There should be construction drawings and acceptance test reports that bear this out. If the tie is at low voltage, a phasing check can be conducted with a power-rated voltmeter. Post again if you need more information.
Another aspect that should have been considered, if all breakers are simultaneously closed—a “closed transition”—the available fault current on the buses roughly doubles, and the gear should be so intended.