Adding multiple transistors to increase the output power of an amplifier
What you suggest can and does work .If you double up the output transistors ensuring that they share with for example seperate emitter resisters ,Then you can expect double the power into half the load resistance .Remember that your power supply voltage is the same but the amplifier puts out twice the current.The power supply will draw twice the current so it must be beefed up too .This has been done on commercial gear where the normal Amp is specified for 4 ohm load and the higher power version with paralleled output transistors is specified down to 2 ohm load .Very high power speakers are hard to find and they have not made 2 ohm speakers since the valve days .Common speakers these days are 4 or 8 ohm so such an Amplifier could run two 4 ohm or four 8 ohm speakers in parallel.