Can I assign IP address on the same subnet on the Magelis HMIG3U using Vijeo Designer V6.2 SP6

No.  The IP address for Ethernet 1 and Ethernet 2 on the HMIG3U must be on different subnet with Vijeo Designer V6.2 SP6 or higher.

SR6 release notes

Cannot configure both Ethernet ports on the HMIG3U on the same subnet
When defining the IP address for the two Ethernet ports on the HMIG3U target machine, do not configure both Ethernet ports on the same subnet.

Released for: Schneider Electric Canada