Once you've created one or more atom selections, you can combine them to create new ones.
>>> CA = AtomSel('name CA')
>>> resid5 = AtomSel('resid 5')
>>> CA
name CA
>>> resid5
resid 5
>>> ANDsel = CA & resid5
>>> ORsel = CA | resid5
>>> NOTsel = -CA
>>> ANDsel
(name CA) and (resid 5)
>>> ORsel
(name CA) or (resid 5)
>>> NOTsel
not (name CA)
>>>
When the combined atom selections are from different molecules or have different frame numbers, the molecule and frame from the first atom selection are used.