I can reproduce the effect on Mac with the X11 version using:
void png_double_x_axis_title(){
gStyle->SetCanvasDefH(2000);
gStyle->SetCanvasDefW(1000);
TF1 *fa1 = new TF1("fa1","sin(x)/x",0,10);
TCanvas *c1 = new TCanvas();
fa1->Draw();
fa1->GetXaxis()->SetTitle("X-axis");
fa1->GetYaxis()->SetTitle("Y-axis");
c1->Modified();
c1->Update();
c1->Print("test1.png");
}
It gives me the attached plot.