What is the ERC (European Research Council)? Find more information here:

-ERC website
-Euresearch: the Swiss guide to European research & innovation

Can I submit to two different ERC calls in the same year (e.g. to the Starting Grant call and the Synergy Grant call)?

Theoretically yes, but it would not make a lot of sense since only the first eligible proposal will be evaluated.
Furthermore, the following rules are applied: A researcher can be PI in only one ERC Project at any one time. And a researchers participating as PI in an ERC project may not submit another proposal, unless the existing project ends no more than two years after the call deadline.