The answer is yes, but with rules to be aware of. (I'm no expert so count all of this as hearsay to verify yourself. There's my disclaimer!) For example, you can't "partner" with your own IRA or lend to yourself from your own IRA. You can flip, wholesale, lend to others, and own rentals in your IRA, but those dollars belong to your IRA, which you can't touch for personal use without penalty until your 60's...standard IRA rules. But just think of the tax advantages when using a Roth account's dollars...it's huge!
So, to do deals where you could access the money earlier than retirement:
Party A can lend my IRA money to Party B, B can lend yours to A, and each can each take that money to use in their own ventures, paying back the loans as agreed. You can't lend to anyone you're closely related to, so no monkey business there. (Makes sense why you'd want to go to mixers and meetups hosted by SDIRA custodians, right?!)
I've done two flips using funds from another investor's self-directed IRA. He gets first lien position and a better return than mutual funds offer. i get private financing cheaper and with fewer hoops than hard money. He uses a custodian out of Waco that he likes. I have my money with Quest IRA out of Houston. If nothing else, go to Quest's site and read their articles, which are really helpful.