I speak in general terms when I say that line outputs of an amp can be used to feed a line input of another amplifier.
But it seems that you are connecting the line out to the instrument input rather than to a line input.
Not all amps would necessarily have a line input. If you are familiar with amps that have effects loops aka effects out/effects in, then those would be a way to use the line in/out connections between two amps. The key is that the audio levels at the output of amp #1 are in the same range as those levels expected by the input of amp #2. If they are not, then you will either have distorted audio or audio that is too low to be usable. The trick in all of this is to match levels between devices. In other words you want the following.
Line level out is here --- and line level in is here ---
Not here ---
and here ---
If you don't have effects loops ins and outs on both amps, it becomes much trickier to get the ins and outs to play nice with each other. Line outputs are not only defined by the audio levels, but also by the impedances of the output vs the input. With effects loops, most everyone agrees on one standard for levels and impedance and this is why using them to connect two amps often works.
As far as your speakers not moving much. This does not really have any bearing on the audio that is passing between the two amps. The line out could be too hot for the line in and cause the audio to distort, but you may not be driving the power amp with enough signal to give you much speaker movement. It gets quite difficult to know for sure what all is going on from where I sit. But the main thing is that when you pass from one amp to another, the input of the second amp requires a certain range of audio characteristics. If the first amp isn't up to the task then things start to not work like you would hope.
