By Paul Taylor VIENNA (Reuters) - Overshadowed by Europe's twin dramas over refugees and Greek debt, debate is bubbling up over how to strengthen European monetary union after six years of debilitating crisis. It pits those who believe the 19-nation euro zone needs to take a bold step towards federal integration to survive as a stable currency area on one side, against those who argue that with scant public or political appetite for sharing more sovereignty, Europe just has to keep muddling through with modest, incremental change. The heads of five European Union institutions proposed in June a series of steps over the next two years to reinforce the bloc's banking union, better integrate its capital, energy and digital markets, foster economic competitiveness and counter macroeconomic imbalances without changing the EU treaty.