Quote from GreenTraderTax:
Your spouse can be a minority owner of the husband wife trading partnership. For example, your spouse can contribute $3,000 to a $30,000 joint individual trading account. That would make her a 10% owner if you base ownership on capital contributions (a fair way to do it, although, other ways are possible as well).
If the minority percentage gets to small, the IRS might think the partnership is more about trying to avoid taxes and then try to block it.
Remember, it is not required that your spouse be "active" in your trading business, but if not, you do need the other factors in place (joint names on account and spouse contributing to capital).
If your spouse if "passive", then your spouse still gets all benefits except spouse has investment interest expense rather then business interest.
It is not hard to establish the factors needed to make this work well for you.