You can chain the attribute values of showon, using [AND], like so:
Adding more fields to the chain is also allowed.
On a side note, you can use an [OR] clause in the ...
(removed my incorrect answer)
I was looking at the JS that parses these showons and it seems that using more than one AND/OR chain isn't going to produce correct results. Only the last pair's ...
This is what I finally did. If someone knows a cleaner solution please let me know.
defined('JPATH_BASE') or die;