Just to clarify this question about priority dates from a European patent application perspective. Each and every claim of a European patent application has one or more effective dates (yes, a single claim may have more than one effective date).
In this case, say P1 is the filing date of the first provisional application, P2 the filing date of the second provisional application, and P3 the filing date of a European patent application or an international application. The effective date of each claim will be determined, as pointed out by George White (much) earlier, by the subject-matter claimed and whether said subject-matter is completely disclosed (either explicitly or implicitly) in one or both priority applications, or only in the EP/PCT application. If the claimed subject-matter is disclosed in both priority applications, then the claim has the earliest date as effective date, i.e. P1.
Concerning the multiple effective dates in a single claim, that may occur when an OR clause is present in the claim, which results in two different embodiments. There are two situations in which this usually occurs:
- An airplane comprising: A, B, C, and two or three engines.
By way of example, if the first provisional application disclosed an airplane with A, B, C and two engines, then that embodiment has P1 as effective date; if no provisional application disclosed an airplane with A, B, C and three engines, then that embodiment has P3 as effective date. This, in essence, means that prior art documents with publication date up to (but not including) P3 can be cited against the latter embodiment, but if the publication date is later than P1, then it cannot be cited against the former embodiment.
The second situation is like this one:
- An airplane comprising A.
- The airplane of claim 1, further comprising B.
- The airplane of any one of claims 1-2, further comprising C.
Claim 3 effectively claims two different embodiments because the dependency has an implicit OR clause: (i) with A+B+C [depending upon claim 2, in turn depending upon claim 1], and (ii) with A+C [depending upon claim 1]. Now, if the second provisional application disclosed an airplane with A+B+C, then (i) has P2 as effective date, and if the first provisional application disclosed an airplane with A+C, then (ii) has P1 as effective date -but of course, if embodiment (i) was disclosed in the first provisional application, then the entire claim has P1 as effective date.
Following the above example, you could have, for instance, claim 1 with P3 as effective date, claim 2 with P1 as effective date, and claim 3 with P1 and P2 as effective dates.
Finally, at the EPO (i.e. under the European Patent Convention) it is not enough that features A, B and C are disclosed in, say, the first provisional application, but these features must be disclosed as being part of a same embodiment (i.e. it is directly and unambiguously derivable for the skilled person that an embodiment of an airplane with A+B+C is disclosed in said document).