Can we get SR2 free if we already bought SP?

No, more $$ for devs = more cool games like SR2 and cool updates for SP. I'm sure you can make a deal with your parents if you for a week or two you do more chores, they would give you money to purchase SR2.

It Would Be Good. But Face It.
There are 117 000 Odd Players On Simpleplanes. So think Of how much money they Will Lose.

no, you have to pay for both

The devs got to get money somehow.