I don't believe that this scenario works with batch campaigns (I'm not sure why, but I know for sure it doesn't). You can use the a/b test in a trigger campaign and it should work as expected.
If your team prefers to use Email Blasts over Email Sends (the mailbox with this built-in functionality) I'd recommend going "old school" - you'd need to create 2 emails (easy to clone and change the subject line) and then in the flow step you'd add the "Random Sample" criteria into the Send Email step. If you want half to get each your random sample would be 50.
As a whole email blast programs allow for a lot more flexibility and testing than sends which it sounds like you all do - so I'd probably go that route.