Can I simply use multiple turbojet engines to fly supersonic?
is it possible to use just two FJ33 jet engines, so that I would just double the maximum thrust ?

Yes, you will double the maximum thrust on the ground and at rest. You will also double thrust at altitude and flight speed, but that thrust will be much lower than your sea level static thrust. Below is a thrust diagram over speed and for several altitudes for a small turbofan engine with a bypass ratio of 3. It is not the Williams, for which I don't have this kind of diagram, but very similar.

The kink in the low level thrust line is from internal engine pressure limits. At low altitude and high speed the pressure inside the compressor and burners must be limited so they will not be overstressed.
Below 10,000 ft it is not legal to fly faster than 250 kts and supersonic flight is illegal over a lot of areas. In order to achieve legal supersonic flight you need to be high and over unpopulated land. The higher the better, but as you can see from the diagram, thrust drops with altitude. For turbofans it also drops with airspeed. Now it would be interesting to know at what altitude and speed those 9 kN of thrust are required - maybe that number is for a lower altitude and the two FJ33 are indeed sufficient. At the moment that question is unanswerable with the information given.

So I would need a diffusor to bring the supersonic flow down to an optimal flow velocity for the FJ33.

Yes, but that is simpler than it seems. Every engine intake will slow down and pre-compress the incoming air stream at high subsonic flight speed simply because the engine will not be able to swallow all the air that could flow through the intake at high speed. Instead, some air will spill over the intake lips so only the core of the streamtube that flows toward the intake will be ingested. At supersonic speed the intake will do this precompression by means of a shock, the strength of which is again influenced by the swallowing capacity of the engine. Still, the intake cross section needs to widen between the intake face and the engine face and must be carefully shaped to avoid flow separation or speed variations at the engine face (that is actually the tricky part) or you will not be able to achieve the full performance of the engine. A simple pitot intake should be sufficient but it will need some length to do its work properly. Therefore, it is advisable to not put the engine in gondolas (B-58 style, also called nacelles) but to integrate them into the fuselage.
Thrust is the difference between intake an exit impulse, so you need high exit speed in order to have useable thrust at supersonic speed. While a simple convergent nozzle can be used to achieve supersonic flight, the best results are obtained with a convergent-divergent nozzle. Due to the high temperature of the exit flow, even a subsonic exhaust stream will bring you to supersonic speed, but only just. Given the relatively high bypass ratio of the FJ33, if supersonic flight is achieved, the maximum Mach number in level flight will not be much above 1 (if the drag characteristics of the airframe allow even this).