What is the priority to be considered as the deadline for extension of a patent application?

Unfortunately, by publicly disclosing your prototype at the science fair before you filed a patent application, any patent application - being provisional or non-provisional - has prevented you from obtaining any foreign patent rights you may seek. In the US, however, you have one year grace period from the date of your science fair disclosure, which, of course, you have already met by filing your initial patent application. Of course, since you seem to be constantly improving your device, system, whatever, whatever you did not disclose publicly at the fair can be foreign-filed, and the one-year dates run from each patent application. So, let's say your original patent application disclosed Invention 1 which you disclosed at the science fair. You will be protected in the US but your foreign rights are gone and cannot be recaptured. You then made some design changes and filed a patent application for Invention 2. You did not say that you publicly disclosed Invention 2, so I will assume that you did not, and therefore you can obtain foreign patents for Invention 2 if you file, for example, a PCT application within one year of filing your US application for Invention 2, provided that you only claim Invention 2 because you cannot claim any foreign rights to Invention 1. The general rule for filing foreign patent application is that for each patent application for each invention, you must foreign file (PCT or within individual countries) within one year of the US filing. You said that after two (2) months from filing your patent application for Invention 1, you filed your patent application for Invention 2, and now it has been 10 months since that filing - therefore, you only have 2 months left to foreign file for Invention 2 through, for example, PCT. This deadline can be extended for 2 months, but this must be done by petition, and there is a petition fee - the petition effectively requests that the right of priority be restored.

One must consider that there are two inventions A and B that follow in time, even if the second (B) is close to the first (A) and consists in its refinement, improvement somehow.
The disclosure of the first invention (A) is not - or more - to be considered in terms of "priority", even though it may weaken the second invention (B) from the viewpoint of patentability (depending on of the applicable law), reason why your attorney advised you to mention it in the first filing (I assume that it is a US patent application?).
If I understand you correctly, the second invention (B) was the subject, only two months after the first (national) filing, of the (national) filing of a second patent application under the (internal) priority of the first one.
Since the first patent application has not had the time to be published (the publication of an application takes place only at the end of 18 months of the "secrecy period"), this means that the "good" priority to be considered is the first filing date (first priority) and that you must file your PCT patent application (International extension) before the end of before the end of the twelve-month period - you mention rightly - from the first priority (and not the second one).
This leaves you very little time if ten months have already elapsed between the two successive national filings!
You will keep the priority of the first application and you will necessary "win" two months in time (this is the advantage of the "internal priority").
But be careful, you have to abandon the first patent application before its official publication by the relevant patent Office because it may "interfere" and "pollute" the second one from the point of view of its patentability (novelty, inventive step). The first national filing has, anyway, no interest because of the prior disclosure of the invention (A) covered.

To sum up:
First, hurry up to place your PCT patent application (WIPO) under priority of the first priority if you want to keep the date of the first filing and thus "win" two more months of prior art,
Secondly, give up the first (national) patent application before the end of his eighteen months (you have less than six months to do).
Please revert to me if you still need clarification.

Sincerely yours,
Simon