Two 64GiB DIMMs would be the more likely setup. The current CPUs strongly prefer having only one stick of DDR5 per channel.
The effectiveness of rowhammer depends on how well the manufacturer implemented target row refresh. But the internal ECC on DDR5 should help defend against it somewhat.
Personally I've been in the 24-32GiB range since 2013, and that's despite the fact that I'm still on DDR3.