Searching multiple texts at the same time is certainly possible, but not in the Combined Texts pane. The hit statistics from the two corpora can also not be combined (except manually).
To Search multiple Texts, try using two linked zones. The first zone will have the Search Argument in the Entry Field, the second will be linked using the [LINK ?] command as its Search Argument, where "?" is replaced by the title of the first zone. That way, anything you search for in the primary zone will be replicated in the secondary zone. [I now see Ken has uploaded a screenshot of this method in his post.]
Another way to do this is to make a custom My Group in the Library. Place all the texts in it you want to search (e.g. GNT28-T, LXX1, Philo-T, APOCAL-T, etc.). Select the Greek word in any tagged text, use a right-click to access the Contextual Menu, then select the proper folder in My Group. Accordance will search every resource in that folder.
Hope this helps!
Edited by Timothy Jenney, 28 February 2014 - 09:08 AM.