HDMI 2.1 cables are backward compatible, which means you can use them with devices that don't support the 2.1 format. However, you can't take advantage of the 2.1 features unless you have a compatible device and TV. For example, you can plug a PS5 into a TV with an HDMI 2.0 port using an HDMI 2.1 cable, and it will work fine. Offset engine pulses scavenge exhaust from the other cylinder for a 2-into-1, this does not occur for 2 into 2. Two into one is a better performing pipe all around. Here's a video by S&S about drag pipes. They show exactly what happens with drag pipes to your performance. V&H ProPipe works great, but there is some low end loss but huge mid-high .

2 into 2 vs 2 into 1