- two aircraft flying in formation more than double the individual RCS

theoretically the total rcs from a collection of scatterers is the square of the phasor summation of the square-roots of the rcs of each scatterer...

so for two objects the maximum (ie. coherent sum) of the echo cross section would be (given: a=rcs of 1st object, b=rcs of 2nd object):

(a^1/2 + b^1/2)^2

if both objects had the same rcs value (ie. a=b) then the maximum coherent sum would be 4 times the rcs of the individual object...

if one object had a fractional rcs as compared to the other object's rcs, ie. b=k*a, where 0<k<1, then the coherent maximum would be:

a * (1 + k^1/2)^2

so if for example your "stealth" tanker had an rcs of 1 sqm and your fighter had an rcs of 0.001 sqm, the maximum coherent rcs of the two combined would be: ~1.06 sqm, or just a bit over 6% higher than that of the tanker alone...

if you got your tanker down to an rcs of 0.1 sqm together with the same 0.001 sqm fighter, then the coherent rcs of the two combined would be 0.121 sqm, or around 21% higher than your tanker alone...

one can also add in the rcs of the boom as a 3rd object in the system and work the math to figure out what ratios of rcs for each object in the system would be needed to get an acceptable rcs increase for the combined system over that of the tanker alone...