That is living on the edge. PCB traces will heat considerably at 50A no matter what. 2W dissipated in a single shunt may cause it to overheat because the traces connect to it don't work as a heatsink so much.
Oh, who wouldn't like living on the edge?
![Fighting :box:](https://www.eevblog.com/forum/Smileys/default/fighting0030.gif)
Depends on how much PCB real estate you have to waste. OP seemingly wants to have multiple sensing circuits, if the space is limited and they want them side-by-side, then they may be out of luck or at least require some very clever tricks to get rid of the heat.
OTOH, if you have ample space around the shunt, you can play the simple trick of doing a "track" which is wider than it is long. With minimized distance, there is not much resistance for the current to flow, yet the extensions sideways pull the heat out of the shunt. An example would be a 40mm x 10 mm fill, where the current flows through that 10mm dimension.
I have played the trick of adding a large bunch of thermal vias around the pads, then add fills on the bottom side, make the bottom side with no component load, add thermal pad and bolt the PCB on a heatsink. You can model this on a napkin or Excel. An example calculation:
Fill area = 10*15 mm = 150mm^2
Num vias on the fill = 12
Rth(via d=0.4mm) = 120 K/W (taken somewhere online)
Rth(12 vias) = 10 K/W
Thermal pad conductivity = 4 W/mK
Thermal pad thickness = 0.5 mm
Rth(thermal pad) = 1/(4W/mK)*0.0005m/(150*10^-6 m^2) = 0.83 K/W
You can lower the Rth(via) slighly, maybe by some 20%-30% again, for no extra cost by opening paste mask on the untented vias, but don't count on this, solder flowing in the vias fully can't be guaranteed.
Resistor datasheets indeed assume some specific footprint for their ratings. In a 2-layer design, you easily end up having to derate as you say. OTOH, on a 4-layer design the chances are you can use the full rating because the thermal conductivity of the 0.1 to 0.2 mm of FR4 prepreg is quite low, and ground plane beneath softens the hotspot. Most importantly, regardless if you simulate it with some expensive package, or do a simple napkin calculation, build a prototype and measure. Since actually measuring the resistor element temperature might prove difficult, I also recommend doing an overload test. For example, if your expected dissipation is 2W, make the resistor dissipate 4W over a full day or two, let it cool and verify it still has correct resistance.