A bank of observers is usually needed to detect and isolate faults (FDI) with robust observer-based approaches. In order to improve the real-time property of the FDI system, the total order of the observers should be designed as small as possible. In this paper, two means that contribute to the reduction of the observer orders are studied. First of all, an approach to the design of minimal order robust FDI observers for given fault sensitivity conditions is developed. The minimal observer order is revealed as a function of a minimal row index of a matrix pencil deduced from known system matrices. Then, the possibility and approach of realizing multiple fault sensitivity conditions with a single robust FDI observer is addressed. A sufficient condition is obtained. The decision logic design is also studied in this paper. A sequential decision logic is proposed, which can be applied to time critical situations where running all the FDI observers in real time is not possible. By revisiting an example taken from literature, the advantages of the presented techniques are illustrated.