Yes, it can be done. Sunn used to put out tech papers, and one was instructions on how to install a line out jack. I haven't seen anything like that for a long time, but I think we can figure it out.
First, you'd need to drill a hole and install a jack. Alternatively, you could build a type of DI box, but recognize that the voltage would change if you changed the transformer tap you used, which happens automatically when you plug into different output jacks.
Next, determine the voltage needed. I think most pro amps will give full output with a 1 volt input. If that's true, you'd want at least 1 volt from the line out. 60 watts into 8 ohms requires 22 volts. So, if you put a 12Kohm resistor in series from the 8 ohm tap on the transformer to the jack, and a 600 ohm resistor across the jack, that should do it. If you put all this in an outboard box, you also want a 6K dropping resistor for when you're using the 4 ohm jack, and a switch to switch between the 12K and 6K resistors. Alternatively, you could use two 6Ks in series, and short one of them out for 4 ohm operation, or two 12Ks in parallel and switch one out of the circuit for 8 ohm ops.
Then, you run a line from your line out jack to the slave amp, and from the slave amp to your second cabinet.
Or, you could just mike the first cabinet!
