You can create a second data frame to act as an insert to your main map. Each MXD file starts with one data frame, called Layers. You can create additional data frames under Insert > Data Frame. The new data frame can contain the exact same layers as the original frame, or you may choose to add or remove certain layers for the insert. You can either copy the layers from the original data frame, or add layers to the new data frame using Add Data. To copy layers from data frame to another, right-click on the layer and select Copy. Right-click on the new data frame and select Paste Layer.

Once you have the new data frame created, switch over to Layout View. You will need to resize each data frame on your page to fit without overlapping. You can do this by clicking on each frame and resizing using the resize handles.