1- Is it possible to make multiple intentions for a single action? For example, I have to make up 2 fasts from ramadan because of travels, if I choose to make it up during next month, could I make the intention of making them up, as well as making it 2 fasts that are recommended of 6 for the month of shawwal for the reward as having fasted the whole year? (As in have both the intention of making up fasts, and doing the recommended)

2- I have done what I mentioned in previous question before by fasting the 13, 14 and 15 of a month. I fasted because I wanted the reward as having fasted the whole month, but I also made intention about making up previous obligatory skipped fast from previous years. Is this permissble, and if not, do I have to make them up once again, by only having the intention of making up the obligatory fast, without the intention of doing the recommended?

1- No. You have to make the intention of Qadha only, and almighty will give you the reward of both.
2- If you fasted with 2 intentions, then the fast is considered void.