Hosting multiple instance of app in a tabcontrol/ queuing problem

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C#

multi-threading

So I have This app with a MainForm that has a few buttons on it. and the buttons would do time consuming tasks that includes working with MainForm UI.
I decided to Create a MainFormHost where it is a form with a tab control which under each tab I create an instance of my MainForm and host it there. And so far everything was ok. The problem is when I click on a button on MAinForm1 it starts working fine but as soon as I click on a button on MainForm2 the process of MainForm1 button gets queued behind the MainForm2 process.

What I have tried:

This is how im creating my Forms

MainForm GetMainFrom(TabPage tabPage)
{
tabPage.Invoke(new Action(() =>
{
    mainForm = new MainForm();
    mainForm.TopLevel = false;
    mainForm.FormBorderStyle = FormBorderStyle.None;
    mainForm.Dock = DockStyle.Fill;

    _mainForms.Add(mainForm);
    tabPage.Controls.Add(mainForm);
    mainForm.Show();

}));
}

And then call the method:

var mainFormThread = new Thread(() =>
{
    mainForm = GetMainFrom(tabPage);
});

mainFormThread.SetApartmentState(ApartmentState.STA);
mainFormThread.Start();

If it can be of any help, Im ok with writing the host app in WPF.