Oh good, at first I thought you were going to call me an idiot. :)
Regarding warfare, there are some rules in terms of peace deals:
-If you are the War Leader for your side, you can make separate peace deals with individual nations. There are some exceptions, such as if they are a vassal or a part of a coalition against you, but in general this is true.
-If you are the War Leader, you can make peace with the opposing War Leader which concludes the war for all nations.
-If you are not the leader, you can make a separate peace deal only with the opposing war leader. Peacing out separately tends to irk your ally a little bit, so make sure your relations are sufficient to take the diplomatic hit.
So in your scenario, you could make a peace treaty with England, but not with Portugal. France could make a separate peace deal with Portugal, though, including granting you land.
Now, I don’t want to bombard you with too much info, but the likelihood of your war leader ally granting you land in a peace deal is influenced largely by a couple factors: The casus belli under which the war is being fought and whether or not you have claims on the provinces you have occupied.
In the case of the HYW, France is fighting a defensive battle against England pressing their claim to the French throne. It is diplomatically expensive for them in this type of war to seize territory from an ally of England, such as Portugal. Because of this, and your lack of Claims on Portuguese territory, the French AI will be unlikely to pay the cost of granting you land in the peace deal. That Portuguese province is going to be 2-3 time as expensive as taking some of the English-controlled territory in France back.
In general, the AI is usually pretty good at making sure you get some land that you have occupied, as long as you have fabricated Claims on it. Of course, to make sure you get exactly what you want it’s best to prosecute your own war and convince France to come and help you. Then you get to set the peace deal exactly how you’d like.