I'm looking to confirm if I need to connect the ground of 2 LED strips which have different power sources.
The key is to have all of the ground pins among the strips connected in common, but the +5V from each power supply should be connected only to one length of NeoPixels — those should not all be joined.
A response to the question When I connect chain of LED strips to power from both ends, do I need to make the power line disconnect in the middle? says:
For a 6m long strip, you will need to power from both ends. It is recommended that you use one power supply (10A should do depending on your LED density) and allow the ground wire to run through the entire length. The power lines in the led strips themselves are not thick enough to carry 10A over 5M without enough loss to cause dimming or color change.
However, Branden's answer in the Powering a WS2812B strip question makes it sound like I should contain the ground to each strip. (Unless I am reading his answer wrong):
So as a suggestion, make sure you power each 1m strip individually instead of tying them all together and having every LED in parallel (and the strips themselves in series). What I mean by this is, tie the +V of all the strips together to one power wire, and then tie all the grounds together. Don't just line up all the strips in a single line, end to end, and solder them. Just the data line should be like this.
I may have confused myself into a corner so I am just looking for a confirmation that the ground needs to be shared across the whole project/all LEDs and all power sources.
Edit: Title change for clarity