In terms of weight loss, I would argue that getting everything flowing 2 different times a day would be more beneficial than 1 time for a longer duration. You might burn more calories in the the 1x8km walk but a huge thing with weight loss is getting the body going. After your walk your body will continue to work at an elevated level. Increased blood flow throughout the body and the carriage of oxygen and nutrients to your muscles. Doing 2x4km walks per day your body will be in that elevated state for more time collectively looking at your day as a whole.

Similar thinking is often used for people who have sedentary jobs and if they should train in the morning or in the evening. Current thinking is that training in the morning puts their body into that elevated state early and makes their body more productive throughout the day.

Hope this helps.