Required Minimum Distributions (RMDs) are calculated differently for distributions from multiple IRAs and multiple 401(k) plans.
If you have more than one traditional IRA, the RMDs are calculated separately for each IRA but can be withdrawn from any of your accounts.
However, if you have multiple 401(k) accounts, the RMD must be calculated for each 401(k) and withdrawn separately from each account. Some 401(k) administrators calculate your required distribution and send it to you automatically if you haven’t withdrawn the money by a certain date, but IRA administrators may not automatically distribute the money from your IRAs.
Source: Financial Planning for Women