I plan to move my SEP-IRA to a self directed IRA or a checkbook IRA/LLC. I have 4x the funds that my wife's IRA has.
Can she set up a self directed IRA and invest in MY self directed IRA? In effect, giving us more funds in one self directed or checkbook IRA/LLC.
Your IRA and that of your wife are disqualified parties to each other. As such, there cannot be any transactions between the two.
The two accounts may joint venture into a transaction. This just needs to be structured in a fashion that both initiate the transaction together and the equity split (say 80/20) is maintained throughout the life of the transaction.
Some folks will create a partnership LLC with multiple IRA accounts as members. We recommend against that as it creates more headaches and limitations than it solves, such as the inability to add more funds in the future and the need to file a partnership tax return for the LLC.