From Pub 590-B:

"Unless you are the decedent's spouse and choose to treat the IRA as your own, you can't combine this basis with any basis you have in your own traditional IRA(s) or any basis in traditional IRA(s) you inherited from other decedents. If you take distributions from both an inherited IRA and your IRA, and each has basis, you must complete separate Forms 8606 to determine the taxable and nontaxable portions of those distributions."