I believe you are seeing decay in what you are calling the "split".   When branches are connected at tight V shaped unions they are weakly attached, can split and decay forms inside them. 

As for branches dying it could be many causes including shading of lower branches (a natural occurrence).  Be on the look out for Japanese Longhorned Cedar Beetles, a new invasive pest that is killing Arborvitae and other cedars.

I would suggest contacting your local Penn State Extension Office and/or contacting an ISA Certified Arborist to evaluate the plantings.

Hello, and thanks for using the Ask an Expert System,

Before you apply insecticide or fungicide it is important to determine if you have insects or a fungal infection. Next it is important to identify the specific pest, and use a pesticide designed for that pest. I am attaching a link to a publication from Cornell Extension on dealing with problems of Arborvitaes.  www.ccenassau.org/hort/fact_sheets/e225_arborvitae_probs_oct00.pdf

I suggest that you contact the Hort Desk in your county. You will be able to discuss your problem with a Master Gardener and provide additional details about your tree’s symptoms and environment.  They may ask you to bring in a sample for identification of the disease or insect. In addition you may need to contact an ISA Certified Arborist to make a site visit to evaluate the plantings.

Hort Desk is a free service offered through the Northampton and Lehigh County Extension Offices. Master Gardeners answer questions on a variety of topics including growing vegetables, fruits, ornamental and turf grass, plant identification, pruning, and insect and disease problems. Advice encourages the use of integrated pest management and environmentally sound practices.