Sure, why not? Here's how my emergency fund is set up:

• 1 months expenses next to my checking
• ~50% of the rest in Ibonds
• ~50% of the rest in t-bills

With ibond rates going down, I'm laddering into t-bills, with the ultimate goal being some bonds maturing every 2 weeks.

I currently also use my brokerage account as my bank account, which is invested in SPAXX (4.91% 7-day average APY). It provides an awesome debit card (no foreign transaction fees, ATM fee reimbursement worldwide, etc), earlier access to my money vs other accounts (e.g. I get paid on Friday, money is there on Thursday), and I've had no issues with it for the year or so I've used it this way. My t-bills live next to my savings "slush" in the same account, so it's quite nice. Here's my actual setup:

• Fidelity Bloom Save - receives paycheck and transfers automatically bimonthly to "checking"; also has my t-bills
• Fidelity Bloom Spend - main "checking" amount, has a debit card, which I never use and keep deactivated
• Fidelity CMA - $0 balance; has my "real" debit card (the awesome one with no FTF), and I just make a transfer from Bloom Spend to this as needed
• Ally Checking and Savings - old main account; connected to Target debit for that sweet 5% discount, and used for a couple of transfers I haven't bothered moving to Fidelity; I get 4% APY on my savings there, but I only keep about a few hundred there so it's not an interesting amount
• Treasury Direct - rest of my efund in Ibonds; I'm considering moving this to t-bills at Fidelity, but I'm still deciding