If I connect my µUSB bus to the other 5V bus what are the consequences? What can I put in place to make it safe?
The µUSB would be plugged in while the circuit is active to reach a µC by default. The other two states are that 5V will be off and 5V will not physically be present because it will be disconnected from the other PCB.
"Or" the supplies together, or shut down the 5V supply if the USB is plugged in. Here is and example of how to share power supplies OR-ing power supplies (diode or mosfet)
If you aren't going to use the power from VBUS on usb then don't connect it at all (you may need to connect it to the a processor or IC for VBUS detection)
If I connect my µUSB bus to the other 5V bus what are the consequences?
If there are any differences in voltage between the supplies then one will feed the other, take for example if VBUS on usb is 4.9V and the +5V rail is 5.01V. You will get an indeterminate amount of current transfering from one to the other dependent on the series resistance of the line and the supplies. Sometimes they even out, sometimes they don't, its anybody's guess. So the best thing to do is use a power sharing scheme or just not use VBUS.
