It's a phase issue. Years ago I tried linking two Vox AC 30's... It sounded like a fart into a sofa, weaker than the one on its own. Basically the frequencies cancel each other out. It's a strange problem. I bet you thought you would sound like God moving furniture!? I later worked with a band who's singer guitarist linked two speaker cabs. We invested in this... https://lehle.com/EN/Lehle-Little-Dual .. It shifts them out of phase and 'voila' well and truly up off the sofa, cocking your leg and letting rip loud enough to blast a hole through your breeches. The sound was immense.