"Don't see how you can have a single tube set up, Jacob! As you described, you've got 2 tubes -- tube A and tube B. The question is which one to choose for double duty as hydration tube, so you can avoid carrying a third tube, correct? It seems pretty obvious that between the two, A and B, one would choose B -- the clean tube, to "multi-use" as hydration tube. And to put on the pack and position the bladder to measure out the correct length first -- as hydration tube -- before cutting. And once Tube B's length is determined, than one can determine the desired length of Tube A as part of the gravity set up."
To reiterate, I never said that anyone carrying a setup like you described, and I currently have, would choose to use a dirty tube for their drink tube. This would require disassembling the whole works and switching the tubes around; that's just madness.
Having said that - yes a single tube setup is possible, you're not thinking outside the box ;) One way is to remove the drink tube cap (with tube attached) from hydration bladder, screw it onto dirty bag, pull bite valve off other end and replace with filter. Point filter output into clean water bottle.
Ok, so that's a major PITA. Another way is to use the quick disconnect fittings supplied by Sawyer to make the tubes and bite valve couple/decouple easily. The new SP122 requires the use of at least 2 of these fittings anyway (they weigh 0.10 oz). To be semantically correct, yes, there are still 2 tubes however one of them can be extremely short. If the short tube is on the clean side (to maximize static head), then presumably the drinking tube is on the dirty side. If the other way around, such as shown in Tony Wong's review of the Sawyer filter, contamination is not an issue unless one decides to put that short tube in their mouth for some odd reason... however this produces the least amount of static head.