In the AFNI user interface, there is only a clumsy way to do this, via the InstaCalc controls.
- Open the Define Overlay control panel
- In the upper right corner is a drop-down menu with the word "Clusters" -- click on that and switch it to "InstaCalc"
- Below will appear a button "Setup ICalc" -- press that button
- A biggish control panel will open up to let you calculate a new overlay volume from 1 or more input volumes.
- Select row A, then click on its "Choose Dataset" button (leave the "Dataset: Value" button alone), then choose a dataset to be "A". If you want a particular sub-brick (besides #0), use the next item in the row to select the index.
- Repeat for row B (you can use the same dataset as A, and pick a different sub-brick index if you like).
- Enter a mathematical expression involving variables A and B in the "Expr" text entry field at the top of the form. For example, the expression A+B will add the two datasets together.
- Press the "Compute InstaCalc" button.
The real trick is figuring out what mathematical expression you want to use to combine the datasets. Without more information, I can't provide much guidance here.
