Is it possible to disable/enable reading from a keyboard connected to
a USB port?  How?

Many thanks,
-Itai

In the same context, if I have two keyboards, can I send commands
sequence to a specific keyboard?  How?
Many thanks,
-Itai

>Is it possible to disable/enable reading from a keyboard connected to a USB 
>port?  How?

unplug it 
it can also be done by Software

>if I have two keyboards, can I send commands sequence to a specific 
>keyboard?  How?

a Keyboard can send but not receive.

tell us what is your Problem which 2 x Keyboard ?

Jim,
In addition to the standard keyboard, there is a scanner connected to
a USB port that operates as a keyboard (scanning a credit-card or
driver-license simply send a long string to the keyboard buffer).  I'd
like to enable the read from that scanner only after prompting a user
to perform the scan and then disable it to prevent a user from
scanning unexpectedly.

hi,

as i say you can disable USB using @Kernel32:DeviceIoControl.

not sure how your Scanner work but my Scanner must be "enable" to use it.
i know those cheap Scanner act as Keyboard-Bridge but i dont like them as 
User can producet exact that Errror you are talking about.