Who decided that Ukraine can't at the same time be in the European Union–Ukraine Association Agreement and in the Eurasian Customs Union?

When trying to research the start of the Maidan Revolution, I see different claims. Supposedly, the EU declared that Ukraine can't join the European Union–Ukraine Association Agreement if it joins the Eurasian Customs Union.

On the other side, there are claims that Russia declared that joining the Eurasian Customs Union means that Ukraine can't join the European Union–Ukraine Association Agreement.

How was the reality at the time? Who made demands about exclusivity?