我可以将同一内容页与两个不同的母版页一起使用吗?

是的,您可以在“内容页”页的“preinit方法”中动态设置母版页:

private void Page_PreInit(object sender, EventArgs e)
{
    this.MasterPageFile = "MyMasterPage.master"
}

设置一些逻辑以动态选择要传入的母版页文件名,现在您正与许多母版页共享一个内容页。

把共享内容包装在 user control ?

A user control is a kind of composite control that works much like an ASP.NET Web page—you can add existing Web server controls and markup to a user control, and define properties and methods for the control. You can then embed them in ASP.NET Web pages, where they act as a unit.