Do I need to aggregate all 401k plans when figuring out taxable portion of Roth conversion?

For example

Client has 2 seperate 401k plans. One contains both after tax and pre tax monies and the other just pre tax money.

If I convert only the one with both pre and post tax money do I need to consider the other one as well in calculating tax?

I understand that if I am converting an IRA I need to use pro rata rule using all IRA accounts.

Is same true for 401 k plans or does each plan stand alone?

Thanks as always

Howard