It is typically safe to use more than one power supply, what isn't safe is not knowing what went wrong with 4pin cpu connector. If the 4pin failed which is 12v and it wasn't on a seperate rail then there is still something wrong with the 12v line in the main (500w) so using it is not safe. I'm not familiar with any power supply which puts the 4pin cpu connector by itself on a rail.
The 4-pin failure was a physical failure, the wires came loose from the plastic connector, no previous power/shorting issues.
I'm currently using it with no problems in an older motherboard that does not require the dedicated 4-pin (my main board won't POST without it). The older board takes a 20-pin connection - the PSU is 20+4.
I've seen a few posts discussing two power supplies possibly interfering with each other, causing ground loops and other current issues, say when used to power motherboard and graphics card respectively. Has anybody here powered/powers a CPU-dedicated 12v with a separate PSU?