Oh I get you, yeah the Greeks would need to amend their constitution as I don’t think they allowed duchies for their princes aha
I think the Grand Duchy of the Peleponnese (or whatever we use) would be a unique title drafted in the Treaty of London to distinguish the designated secondary line.
As such his children would be 'of the Peleponese' with the senior male always being the Grand Duke and the second male as simple Duke - and all subsequent simply being ' of the ...'
Leopold of Saxe-Coburg and Gotha / King George I of Greece
m. Louise-Marie of Orleans / Queen Marie of Greece
Issue:
1 - Crown Prince Constantine of Greece (later, King Constantine of Greece)
2 - Prince Leopold of Greece
3 - Prince Philip of Greece
4 - Princess Charlotte of Greece
Prince Otto of Bavaria / Grand Duke Otto of the Peleponnese
m. Amalia of Oldenburg / Grand Duchess Amalia of the Peleponnese
Issue:
1 - Duke Ludwig of the Peleponnese
2 - Adalbert of the Peleponnese
3 - Mathilde of the Peleponnese
* - Mathilde and Charlotte's children would then drop the 'of ...' designation which would continue through the male line only as both Russia and France followed Salic law of some form or other.