Economics and Government are usually each a semester course, so they are often done the same year. It doesn't need to be in a specific grade level. Though it makes sense to cover both Economics and Government in 12th, it can make sense to teach them a different year.

The information in the class is helpful in 12th grade, because the student is about 18 years old. They need the information to pay their taxes for the first time (which involves Economics) and to vote for the first time (which involves Government).

Teaching information when it's timely can help students understand concepts better. For that reason, it may make sense to teach Economics and Government during an election year. That way they can more fully understand the process of electing (which involves Government). And, they will better understand the discussions on how people choose their candidate (which involves Economics).

This article covers social studies in more detail: Social Sciences and High School History

Important tip for either year you decide to teach Economics and Government:

Regardless of what year you choose to teach Economics and Government, be careful that your overall course planning is balanced. Don't make 12th grade a significantly lighter load. Colleges look at trends. They want to see that students have consistent challenging work. If the student started slowly, then the look for increasing difficulty and challenge. What they don't want to see is anything that looks like 'senioritis' or slacking off.