Please help me understand how the ode45 routine acceses the rows of odefun individually.
It doesn't, not the way I think you mean it.
odefun = @(x,T) [T(2); -a*(Ta-T(1))];
This is not two "individual function[sic] defined in each row in my Runge-Kutta function". This is one anonymous function that accepts two inputs, x and T (which must have at least two elements) and returns a vector as its output. That vector has at least two elements assuming neither a nor Ta were empty, but still just one function.
You could call your odefun with two inputs and then index into the vector that odefun returns, but that does not make odefun two functions in any way, shape, or form.
f = @(x, y) [sind(x); cosd(y)];
Because of the way f is constructed and called, z(2) is cosd(135).