E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
At the same time; in equal degrees; two or more schemes carried on at once and driven forward with equal energy, are said to be carried on pari passu, which is Latin for equal strides or the equally measured pace of persons marching together.
The cooling effects of surrounding matter go on nearly pari passu with the heating.Grove: Correlation of Physical Forces, p. 64.