In general, one should look at their allocation and adjust it on a regular basis, annual is a typical choice. Withdrawals are cash, of course, and after a year of withdrawals, you'd look and see what needs to be done to re-balance.
Keeping two accounts is fine, but you still have one portfolio. And depending how the stock account is invested, it will accumulate cash from the dividends.
There's no magic way to do this. And no magic mix, except, perhaps, for the mix that lets you sleep best at night. As you withdraw your cash, the stock % will increase a bit, as it will 2/3 of the time, when the market rises. In down years, your reallocation might be to purchase more stock.
Note - littleadv asked an interesting point of clarification, worth addressing. Some people are confused regarding RMDs. Obviously, if you need spending money, you can't spend shares of Apple. But, to satisfy your RMD, you can withdraw shares 'in-kind' which means taking the stock as an RMD. Say your IRA happens to have a mix of stocks and your RMD is $5000. You can transfer 10 shares of Apple from the IRA to the non-retirement broker account, and once it clears, you'll see the value it was transferred at. You then might have to take a few hundred dollars more to meet the exact RMD, or you might have gone a bit over. Now you'll treat the RMD as income, and will need to pay tax out of your cash account.