How to select current for power supply?
The current rating of the power supply is not important, if the minimum requirements are fulfilled. 

The current is determined by the output voltage and the impedance of your load. When a power supply is rated for up to 4.5A then this is the maximum that this power supply is capable of. It does NOT mean, that the power supply is always outputting this current. So, also your load might only need 2 x 0.4A, a 4.5A power supply will do no harm to your devices (because there will be only a current flow of these 0.8A).

But I'm not sure why you think you would need double the rated voltage. A voltage exceeding the ratings of your devices WILL damage something!