A question about sizing and placing fuses
The challenge with using fuses is that they can handle loads larger than their rated value for extended time without blowing. It depends on the class of fuse, but most will carry 125% of full load indefinitely without tripping (at room temp). Datasheet for fuses will show I squared t trip value, which is how long it will take for fuse to blow based on the square of the current. They are also temperature sensitive since they are thermally driven devices. When you say you do not want PSU1 and PSU2 to exceed 500mA, is that on line side or load side? 

If you are trying to limit AC input to PS, then you need over-current (OC) protection on line side. A fuse will prevent catastrophic failure of the PS from causing overload on conductors - but keep in mind than small overloads will not trip fuse. Basically, it will provide good short circuit protection and prevent fire hazard. There is no simple easy solution to limit input AC current with any degree of precision. 

If you are trying to limit output of power supply, you have a couple of ways to go.

1) Select power supply which has fixed or adjustable output current. Under over load conditions, these PS units will reduce the output voltage until the current no longer exceeds the set limit. This method can be very precise. There are low cost off the shelf PS with 99% accuracy current limiting

2) Place fuses in output of of each PS. Again, this will be much lower precision, but will work just fine.

Also PS units that are UL or CE rated must have some form form of OV protection to prevent internal failure from creating fire hazard . If using UL or CE rated PS, I would consider adding fuse at input to box to protect against any type of internal wiring fault, as well as any other type of catastrophic failure. You can use a chassis mounted fuseholder with glass or ceramic fuse

Good luck.