Hello. Welcome to AA!
Though it is possible to cycle in one week, it would require large amounts of seeded filter media from an established tank. A few decorations from one will definitely help speed up the process but won't make it that fast.
Unfortunately I have some bad news about the goldfish. Because of their expected size and the amount of waste they produce, keeping them in a 28L tank, even for the short term, isn't a viable option. Goldfish produce large amounts of waste for a fish of their size. Because of this they require at least 80L of water for the first fish and 40L for each after that. So for your two your looking at a 120L tank at the very minimum. I know it sounds like a lot but the larger the tank, the easier to keep the water conditions clean.
I would suggest that you invest in your own liquid test kit. A kit is an essential tool when it comes to cycling a tank. Water parameters can change in a matter if hours in over stocked, cycling tanks.
Here is an article one of our experienced members wrote for another site.