One of your colleagues should be able to see applications from job openings on organizational unit A and B?

There are two ways to implement this.

A) The user receives one additional account/user

One account is created on the organisational unit A and the other on B.
In order to process the applications from both job openings, he/she must log in to the different accounts.

B) A new organizational level is created

This must then be above the two required and the user relocated to it.
The user can then see the applications of all job openings that are below the new organization level.