Is the relationship between T30 and T60 linear, so that T60=2*T30?

No, all the methods for measuring reverberation time in WinMLS gives the time it takes for the sound to decay 60 dB as result. In a normal room, this decay is supposed to be linear as function of time (a straight line). We can therefore find the reverberation time by measuring 30 dB decay and extrapolate the rest of the line down to 60 dB. This is what is recommended in the ISO 3382 standard and is useful in cases where it is difficult to get a signal-to-noise ratio that is better than 60 dB.