I'm going to use a Raspberry Pi and a Pico Pi plugged together. They are both powered by the same 5V power supply (thru the 5V pins for the Raspberry, and thru VSYS for the Pico).
So far, I've used a single 3V3 net to power other components, which is tied to the 3V3 pins of both the Raspberry and the Pico. Is this something I should avoid, and if so can you explain why?
(The 2 of them are also directly linked via UART RX/TX).
To connect two or more voltage regulators in parallel you must work on ensuring load sharing.
In fact, if the voltage is regulated while currents are not well shared, one regulator will get heated more than the others to get overheated and may not survive and hence could draw others with it for the same reason.
This Application Note explains different methods used for load sharing.
In your case I think it is better to separately supply two groups of components.
Why tie the 3v3 together? This should be avoided unless you have a specific reason like wanting to run a failsafe configuration, in which case you'll want to use schottky diodes to prevent back current in the simple case, or a PMIC if you want to be more thorough. The easier thing to do is just use one. Look up the schematics for both Pi boards. They will both have a 3v3 regulator probably integrated in a PMIC, and you don't want the two to fight with each other.
