How do you add two possibility frontiers (from different regions) together to find the total frontier for the whole country?

How do you add two possibility frontiers (from different regions) together to find the total frontier for the whole country?

Information is as follows:

Xa = Lx^(1/2)

Ya = Ly^(1/2)

Xb = 1/2Lx^(1/2)

Yb = 1/2Ly^(1/2)

Frontiers are:

Xa^2 + Ya^2 = 100

Xb^2 + Yb^2 = 25

Thank you!