Yes, it is possible to apply for an E-2 visa while already in the United States on a different visa. However, it's essential to understand that this course of action is not typically recommended unless you have no other options. The reasoning behind this advice is that when you apply for an E-2 visa from within the United States and get approval, you only receive E-2 status, not an actual E-2 visa.
The critical difference lies in travel privileges. An E-2 visa, like other visas, serves as an entry document to the United States. Without it, if you leave the country for any reason, you will lose your E-2 status and be unable to re-enter.
Despite this, some individuals in the US on different visas such as tourist or student visas may choose to apply for an E-2 visa from within the country. It's worth noting that this route usually incurs more costs and comes with the significant drawback of not receiving an actual E-2 visa, especially if you have frequent travel needs. For more information on Best US Visas For Immigrant Entrepreneurs, an initial consultation is your next best step.