Subs which have their own amplifier, and have speaker-level inputs, draw essentially no power from the main amplifier. They are designed to accept speaker-level voltages, which are much too high to feed into a normal power amplifier input, and to generate the high current and power levels required by their driver, in proportion to that input voltage.
To put two amplifiers in series, you would have to provide a means of reducing the voltage levels coming out of the first amp, or have the second amp specially designed to accept higher than normal voltage levels. More significantly, in either of those cases the power generation capability of the first amp would be completely unutilized and wasted, and the power level that could be provided to the speaker would be the same as the power capability of the second amp.
Equipment which has been available to provide a means of putting two amps in series typically addresses the situation where there is an underpowered receiver, and the user wants to connect a separate higher powered amplifier, but the receiver does not provide preamp-out jacks to connect to that amp.
Another problem that would arise is that an amp that has output transformers, such as most tube amps, should not be operated without a speaker load or equivalent resistive load, or damage can result. Therefore a dummy load, such as a high power 8 ohm resistor, would have to be provided, either internally in the second amp or externally. But again, the power generation capability of the first amp, and the expense that represents in terms of hardware, would be wasted.