You never want to take any sort of output from a guitar amp and run it into the front end of another guitar amp, plain and simple.
A "line out" signal is taken after the preamp but before the power amp. It is much hotter than the raw output of the guitar. It will not power a speaker. You could run it into the front end of a guitar amp, but it would probably sound like moist ass because it's so hot. The proper use of a line out would be to run it into a power amp. If you really wanted to run it into a guitar amp, you could run it into the effects return so it only uses the power amp.
A "speaker out" signal has a lot of power, and should only be connected to speakers. If you plug it into an amp, you will most likely damage/destroy the amp.