I wanted to make sure that my solution would work before wiring it physically and I felt that all the great minds here would be able to do that.
So I have two WS2812B 5 m LED strips, each containing 60 LED/m, or 300 LED per strip. The manufacturer states 0.3 W per LED, and these are 5 V LEDs. That would mean each strip is 300 × 0.3 = 90 W, and requires 90/5 = 18 A at a full white bright.
If I were to just solder one of the 5 m to the end of the other 5 m, I would now have 600 LEDs in parallel correct? Which would mean a current of 36 A would be going through the top line before the respective currents go to the ground after the LEDs. I don't believe the LED strips could handle that much current.
After some testing it turns out that the voltage loss is significant enough that I can't even power one LED from one side, so I'd have to power it from both ends. In order to have two strips powered, I'd think I can power it then from three places, the beginning, the connection of the two strips, and the end of the second strip.
I drew up this quick wiring diagram, would something like this work? I would have three power sources with two grounds, and the LED strips would be fully connected.