Dangers of Dual PSU

None, provided you don't power one component from both the PSUs, so for example all the power to your motherboard would need to come from one of the PSUs only, etc

... and provided you can fit it all in your case with sufficient airflow etc

This'll sum it up nicely - (It is quite long to explain in details)

Courtesy of Phaedrus2129 on overclockers.net

"The vast majority of people who use two power supplies don't need to, and waste money or endanger their rigs doing so. It's only a valid approach in a very small handful of scenarios, and is otherwise either an unnecessary over-complication, or a dangerous jury rig solution.

The main thing people do wrong with dual power supplies is to think they even need it. They look at a GTX580, then look at power consumption charts and think "OMG 500W OMG OMG NEED MOAR POWER" when in fact that's the total system power consumption; the card itself only pulls ~300W. So then they decide to supplement their already more-than-sufficient 650-850W PSU with another PSU, wasting money and making life more difficult, what with having to stash another PSU somewhere inside their case and hiding a bunch more cables.

The next worse thing people do is do dual PSUs with cheap units. Most power supplies under $100, and all under $50, use a technology called "group regulation" to reduce costs. There's a bunch of technical stuff I could throw at you; but the gist of it is that the current on the +12V and +5V rails on a group regulated power supply must stay within a certain approximate ratio to one another (say 3A of +12V for every 1A of +5V, +/-20%) or else the voltage regulation will go out of whack. +12V load too low? +5V voltage drops and +12V voltage soars. +5V load too low? +5V soars and +12V drops.

The secondary PSU in a dual-PSU usually has no +5V load at all, or at most 1-2A, meaning that the +12V voltage will droop significantly; on high-end group regulated units it will fall to 11.6-11.7V, which is in-spec but poor. On low-end group regulated units +12V might drop to 11.3-11.5V, out of spec or almost. Out of spec voltage (>11.4V) can cause component malfunction; bluescreens and random shut downs, and will prevent many hard drives from functioning at all.. Very low, but in-spec voltage (11.4-11.6V) can cause poor overclocking results, occasional instability, excessive wear on component power circuitry, and can cause hard drive issues over time.

Dual PSUs should only be used when both power supplies are high quality and use independent regulation, or even better "DC-DC" regulation. It's only a useful approach when dealing with systems that pull >1000W; then two quality 650W+ indy/DC-DC regulated units may be used in place of a single 1000W+ unit. Even then you may run into issues of crosstalk leading to greater effective ripple on power regulated devices (motherboard, graphics card, RAID controllers), and units that rely on +12V v-sense may suffer poor voltage regulation if the cable with their v-sense wires is not in use."

Overheating if you have poor airflow, and cable management could be a overheating factor if you have poor management or get non-modular units. Other than that, it's just the usual power supply warnings- make sure you get a good unit, enough wattage, but I going to guess that you already know that if you are investing in a powerful enough PC that it requires two separate power supplies. Also, make sure your case has 2 PSU mounts.

For a server that is designed to have redundant power supplies it is a great idea. For a desktop, it is unnecessary complexity and possibility for failure.

Its fine as long as no one component is receiving power from both PSU. If say, you had a working 500W for your current setup, and now want to add a whole bunch of fans you can just buy another unit to power them if your your first unit is not enough to power everything.

Although ideally you just want to have one strong PSU, so that you won't have another one sitting outside the case and adding power cables.