This is generally pointless and dangerous.

Connecting two PSU in paralell even if they are the same model will create a more unstable voltage in most cases.
This is because as the load fluctuates the PSU's will try and compensate to keep the voltage stable.. but they will do this differently and if there is any PD between the PSU then you might get them "shorting" against each other a bit, especially if they cannot equalise the lines (if 12v and 5v are combined on bother PSU this is likely)

This might just mean a less clean voltage, but it might blow up one of the PSU.
If your PSU blows up you might kill other components.

The same is true for using a second PSU to power hard disks or graphics cards, except in this case you may get current flowing through IDE cables or the motherboard between the PSU where only small amperage is permitted... this might kill the HDD, graphics card or motherboard.

The only safe way to run two PSU is to use one for all components that connect to the motherboard and just use the second PSU for fans and watercooling... generally thats not worth the bother.
Or if your powering peltiers, but even then I recomend a dedicated unit.

If your PSU is not doing the job just buy a newer better one.
Messing about with dual PSU probably wont help you... and might kill your stuff so dont bother.