Sorry about the previous answer...not an accurate one (is valid for most of the nodes in hypershade). Here's a better one:

Take a look at the picture below.
- First click and hold on the output triangle (1) of the first bump map node and select outAlpha from the pop-up menu.
- Then click and hold on the input triangle (2) of the +/- avg node and select input1D[n] -> [0]
- Repeat the same for the other bump map. Connect from 3 to 4 (select input1D[n] -> [1] this time)

You can continue adding virtually as many inputs to the +/- avg node as you like. There will be always one free index for the next connection.

Hope this helps...