Are there any tax benefits of investing in stock market via a partnership LLC as compared to individual trading account?

While there are no apparent tax benefits to forming an LLC and combining your individual investment accounts, there is one possible disadvantage - assuming you hold different stocks in your two accounts, when you combine them in a single LLC, you may each have to recognize gain if there is unrecognized gain in your portfolios, as if you had traded (example) so many shares of Ford for so many shares of GM. Check with your accountant or tax return preparer about this problem (called a "mixing bowl problem" ) before you proceed.

As noted by my colleague, this kind of arrangement may cause various tax complications. Generally, people often establish entities to take advantage of limited liability. That is useful when you have a business, rental property, so on that may be subject to lawsuits by third parties. For your joint investing, there is no concern about you or your girlfriend getting sued so having an entity is of no use for that purpose. I see plenty of downsides (not the least of which is the issue if you separate and one decides to remove money from the account) and no positives at all. As advised, you should speak with an accountant to be sure.

No. There are lots of negatives and no reason to do it.