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I would not tie them together. This will invite fungal infections, and you could lose the entire bush. The best thing to do now, is to cut off any growth that you know is dead, and trim it up the best that you can to reshape. It will recover from here, but you will just have to have some patience with it. Do you know, exactly, what kind of shrubs that they are?

Unfortunately, this growth can be hard to promote to come back. The best solution, if after this year it does not branch back out, would be to cut back the growth closer to the ground and reshape them.

It could be too much water, but what I'm wondering is... What is the soil type? It could be way too alkaline if there is a lot of limestone around. Second suspect would be fungal. Either way, both can be corrected with Wettable sulfur. Spread this according to the packaging instruction and it will stop the spread. What is dead, is dead, and will be replaced on the top and tips of the branches.

Unfortunately, the only thing you can do it wait to see if it will recover. This is detrimental to an established tree such as this. They don't tolerate heavy pruning. You should only take out anything dead or dying.

Hm... They don't really feed heavily, so the only thing that I could see happening is that the fertilizer was too strong, or possibly high in urea. This type of nitrogen is hard for plants to take up without the proper soil microbes to break it down into ammonia. I would add a proper mycorrhizae to the soil. A google search will provide many places to purchase this beneficial group of microbes.