Amplifier output to other channel input

So I have seen other people take a 2x15watt stereo class d amp and send a mono signal into one channel and wire in two full range speakers to the output of that same channel of the amp. He also connected that output to a low pass filter and then back to the second input of the other channel. Then wired his sub into that 2nd output channel of the amp.

I was wondering if this is a good thing to do/practice or if it can cause problems. I was thinking about trying it out in like a small cheap diy Bluetooth speaker but wanted to check in here first.