Let's say two users (Us1 & Us2) insert a row into a table containing an identity column (value 1 assigned to Us1, value 2 assigned to Us2).

If the operation of Us1 fails and is rolled back, and operation of Us2 is committed, value of 1 in identity column is lost.

Is there any way of ensuring that there are no "holes" in an identity column?

no,

do you need continous identities with your definied increment?
then you have to write your own package to handle that stuff,
but keep in mind:
if ever any business need deletes rows and you reuse them again
e.g. customerID & then reusing the number for another customer
how will be able to continue SOX compliancy??? if both had/have bookings ....

every customer I know lives with "holes" in idenity columns