I see you are including a vacuum pump with the BROV kit which is a good idea I went through two before I found one that did not leak.
A question, it looks from the store page that you are supplying one plastic hose and one connection with the pump. I have been vacuum testing both housings at the same time. Is one at a time better?
No, it is better to vacuum test both housings at the same time. There can be a very slight leak via the power cable from one housing to the other so it is better to vacuum test both at the same time.
We supply a tee connection and two vacuum hoses with the ROV. That allows you to test both enclosures at once. The normal version sold on our site is set up to test just one enclosure at a time.