They can be displayed ...(together/ all in once ?).... for visual comparison
When you want to say something are displayed or processed or done together..
I agree with most choices in JMB's answer; however, I would personally phrase it differently.
They can be displayed ____ for visual comparison.
Instead of emphasizing on "when" (as in "all together", or "all at once"), I believe that emphasizing on "how" makes the sentence flow better, and clearer. For example,
They can be displayed on the same page for visual comparison.
They can be displayed in the same window for visual comparison.
They can be displayed side by side for visual comparison.
They can be displayed next to each other for visual comparison. (if there are two of them)
They can be displayed next to one another for visual comparison. (more than two)
If the verb is "process", I would use a different word, simultaneously.
These tasks will be processed simultaneously.
They can be displayed
All together
In a group
In a row
All in one area
They can be processed
At once
In one go
All together
Concomitantly- at the same time; simultaneously