Haven't you ever heard that two heads are better than one?
With one module (head) you can test one tube at a time. You have to power down the amp, move the module to another socket, then power the amp up to test the second tube.
There are a number of "how to" books and videos available at Antique Electronics (www.tubesandmore.com
). There is a FAQ here on TB with a section full of links on tube amp repair.