At the same time, is what we do.
As do we. Subtract the ongoing damage from the regeneration, and apply the result to the character's hit point total.
Never, ever, cross the streams. That... would be bad.
It's not specified, so it's player choice. If the ongoing damage would make you drop but the regeneration would heal you up again, do the regeneration first. If the regeneration would be wasted because it would heal you over your max HP, then do the ongoing damage first.
Thank you for those words of wisdom, Dr Spengler...![]()