Second paragraph above is incorrect. Looking at the image above, the blade marked "accessory/device side" does not connect to the upper right fuse socket. If you install the fuseholder rotated 180 degrees into the fuse panel, you will not get power flowing to your additional accessory, with or without a fuse installed. Also, no current will flow through the additional fuse. There is no "overloading" of any fuse any more than if the fuseholder is installed in the correct orientation. Easily verified by the continuity function on your meter.Just a note of caution. When using fuse taps, make certain that you have installed them in the correct direction!!!
If you install them backwards, you'll be drawing your power through both fuses, potentially overloading one fuse.
Also, the Lumision instructions for selecting the fuse in step 3 is conceptually incorrect. Sure the fuse has to pass the current required by the accessory, but the purpose of the fuse is to interrupt the current before the current exceeds the current-carrying capacity of the attached wire. The maximum current rating of the fuse is determined by the current rating of the fuseholder and the downstream wiring to the accessory.