RO/DI storage and salt water mixing are like most things in the hobby - there's an ideal way to do it and the compromises we make due to our personal space and budget.
I live in a small 800 sq ft. space where we keep our 90 gallon mixed reef, so I opted for a larger ATO reservoir and a flexible RO/DI and saltwater mixing 28 gallon brute container. This means I make about 12 gallons of RO/DI for topoff water every 2 weeks to fill our ATO reservoir and a full 25 gallon saltwater batch for a water change every two weeks.
I fill the brute to 25 gallons, top off the ATO reservoir from there, re-fill it with RO/DI, mix a batch of saltwater, do our water change, clean it out and let it sit empty for another few weeks. In our house the RO/DI source is in the laundry room and the Brute container is on wheels - so I can mix RO/DI or saltwater and the wheel it in to our tank to fill up the ATO or do a water change.
It would be better if I had a 100 gallon RO/DI storage container and more space, but I can easily source RO/DI, deliver it, mix saltwater and cure it for 24-48 hours without too much trouble.
So like most things in this hobby - it's awesome if you have space/finances for two large containers for RO/DI and saltwater, but you can definitely make due sharing one container - whether it's a 5 gallon bucket or a brute on wheels.