I believe it is essencial that the Zabbix server provides an option to poll different items at the same time, especially after calculated items were implemented.
In most cases, the items used in a calculated item must be fetched at the same time, or else the calculated value will not be valid. For instance, a hypothetical calculated item that shows the cpu usage by adding the system and user utilizations will be meaningless unless those two items are polled at the same time.
Another example that doesn't involve calculated items would be a trigger "A" that should only be true if a trigger "B" is false. In this case, if the items used in trigger "B" are not fetched at the same time as the items in trigger "A", trigger "A" could be active because trigger "B" hasn't had its items updated yet, even though trigger "B" will also evaluate to true after its items are updated.
In the GUI, Affinity Groups could be created and displayed similar to the existing Group option.