I've used a pair of combos together and discovered that there are some real practical advantages to using some kind of splitter rather than linking them directly. First, unless the two combos are identical, you'll want to be able to adjust their volumes and EQ's separately to get them to play nice with each other. Second, if you use any pedals/effects, you can put them between the splitter and one of the two amps, thereby sending a clean signal to one amp to maintain the bottom end while sending the effected signal to the other. (This is especially useful with "bottom-sucking" effects.) Because I was only doing this a couple of times I didn't want to invest in a dedicated signal splitter, so I just used a Sansamp BDDI that I happened to have around which had two outputs.