In CSS2.1, an element can only have at most one of any kind of pseudo-element at any time. (This means an element can have both a :before and an :after pseudo-element — it just cannot have more than one of each kind.)
As a result, when you have multiple :before rules matching the same element, they will all cascade and apply to a single :before pseudo-element, as with a normal element. In your example, the end result looks like this:
.circle.now:before {
content: "Now";
font-size: 19px;
color: black;
}
As you can see, only the content declaration that has highest precedence (as mentioned, the one that comes last) will take effect — the rest of the declarations are discarded, as is the case with any other CSS property.
This behavior is described in the Selectors section of CSS2.1:
Pseudo-elements behave just like real elements in CSS with the exceptions described below and elsewhere.
This implies that selectors with pseudo-elements work just like selectors for normal elements. It also means the cascade should work the same way. Strangely, CSS2.1 appears to be the only reference; neither css3-selectors nor css3-cascade mention this at all, and it remains to be seen whether it will be clarified in a future specification.
If an element can match more than one selector with the same pseudo-element, and you want all of them to apply somehow, you will need to create additional CSS rules with combined selectors so that you can specify exactly what the browser should do in those cases. I can't provide a complete example including the content property here, since it's not clear for instance whether the symbol or the text should come first. But the selector you need for this combined rule is either .circle.now:before or .now.circle:before — whichever selector you choose is personal preference as both selectors are equivalent, it's only the value of the content property that you will need to define yourself.
If you still need a concrete example, see my answer to this similar question.
The legacy css3-content specification contains a section on inserting multiple ::before and ::after pseudo-elements using a notation that's compatible with the CSS2.1 cascade, but note that that particular document is obsolete — it hasn't been updated since 2003, and no one has implemented that feature in the past decade. The good news is that the abandoned document is actively undergoing a rewrite in the guise of css-content-3 and css-pseudo-4. The bad news is that the multiple pseudo-elements feature is nowhere to be found in either specification, presumably owing, again, to lack of implementer interest.