Chris if the signal chain is stereo with two mono sources, in series, the two available channels are L R, where you can place/pan each mono source. If the block stayed stereo in series, you could pan each source to each side/channel, and they would never touch/mix, it's still series but each source on their own side of the tracks.
if the block was true stereo, and remained true stereo in series configuration, hard panning them to either side they would never touch, a slight pan of each would open up space on the sides where they wouldn't touch, but touch/mix in the middle. given the point of consolidating into one stereo block was for convolution purposes I would have thought stereo operation would remain a priority