Some guys seem to do this so they have their effects through one side and a clean sound through the other. In order to match a very loud guitarist I ran a TH500 through a Berg CN212 and daisy chained my backup TH500 through 2 AE112 cabs. I was only after some additional volume but I was so loud neither me nor the drummer could hear the guitarist on stage, during the sound check. It was an interesting experiment but the overall volume was so high that I had to turn down to the point of defeating the purpose. I could have been that loud by turning up my TH500/CN212 rig. For my sensible loud rig I usually run 3 x 12's powered by my 2 ohm Thunderfunk.
Yup! Two X 4KT88 all tube heads driven with an Ampeg SVTIiP via a Lehle Dual splitter. 8X10 Eden driven with Simms Watts AP200 Super 2 X Ampeg SVT115A for Altec driven with Hiwatt Dr201 clone. The 8x10" with tweeter stays cleanish and the 2X15" without gets dirtyish.
I'm with Bassmec. To paraphrase; I don't always use two cabs with two heads ...but when I do,I use two heads with four cabs. After all everyone knows two heads are better than one!
I have separate outputs from the pickups on my bass. Separate independent signals means no phase fun.