The Line Out is a line level output coming from the output of the power amp. It is reduced in level so that it can be used as a direct out to a PA, recording unit or external power amp. Note, however, that it's a full range output, so, if you're running distortion, you'll get all those nasty high end harmonics which are normally filtered out by the frequency response of the guitar speakers. Modern DI boxes do a similar thing, so their output has a frequency response similar to what you'd get if you miked the speaker cabinet. That's actually what I meant when I said "preamp out." The T doesn't have a pre out, it has a line out.
So, you could run a line out into a second T, but I wouldn't recommend it. Much better to run a cord from one or both OUT jacks to the IN jacks on the second amp. Set the tone controls the same, and you have twice the power. That only makes a few dB difference, though. Better to use more speaker cabinets. The T will power a 2 ohm load. That's four 8 ohm cabinets.
Finally, from what I've read, you got the good Model T. The second generation has the mid selector switch beneath the mid control, and is said to not sound nearly as good. That's the model I have, and it's hard to get a really nice bass sound, though it's okay for guitar. I'm thinking of modifying it to the original specs.
