Earlier this week I had to re-cable a Burley tandem. Both shift cables routed to DT penetrations near the HT then open air to the rear, thru the DT, the front BB shell, the cross tube to the rear BB shell then out the tube opening. UGH, about 4ft of tubing and two shells with no internal housing or conduit or nuttin. "Ah, I read about this on BF C&V"
I got out the long length of tubing that Jagwire sells, just like shift cable liner, translucent, stiff and just big enough to cover a brake cable. Cut the first cable about a foot from the rear tube opening then CAREFULLY fed the tubing over the cable, past the two BB shells , up the DT and, with some fiddling, trial and error out the penetrations at the front. Pulled the old cable out and fed the new cable in. Pulled the tubing and repeated for the second cable. Slow and deliberate but that worked fine. I needed a coffee break after that.
BTW, I've had NO luck attaching a new cable to the old and pulling it through. Tape does not work on old oily, dirty cables let alone the new cables. Whatever you use to connect the two cables, if it does hold, gets snagged somewhere inside the frame.