If you have a 401K and a ROTH 401K you can roll the 401K into an IRA and the ROTH 401K to a ROTH IRA plain and simple with no issues... the 401K plan administrator will issue 2 1099-R forms for each rollover.
Now if you have a 401K with pre tax and after tax contributions in it then you cannot ONLY move the after tax contributions out of the account. It is not normal for a 401K to have after tax contibutions but if it does then you must roll the entire amount of the 401K to the IRA ( the after tax portion will be reported on the form 8606) and then you can make a ROTH conversion anytime you want however you still cannot only roll the after tax amounts ... each conversion will be a ratio of before and after tax amounts reported on the form 8606.
