Interesting.
The reason is that we index gateways by NetbiosName.
Since both servers has the same netbios name, the last installation (GW registration) wins.
Thanks for reporting this, I will open a card internally to track this.
I think it's the first time I see 2 DCs with the same netbios name on the same forest...
Sadly, for now I can't think of a workaround besides changing one of the DC's netbios name.
Eli