When I worked at Google (2010), this was a major pain point. Your @google.com account is essentially a Google apps account and most people have a personal @gmail.com account. At the time, the best solution was to use multiple browsers or incognito mode.

The flows can definitely be improved and I'd attribute this to it being a hard implementation problem rather than a lack of awareness around what a good flow looks like. Most companies don't even try to support letting you log into multiple accounts at one time. Having multiple domains (google.com, youtube.com, Google Apps domains) makes it even more complex.