You can have multiple vCD cells with vCloud, like you're talking about. However, these cells aren't in an active/standby configuration like you're thinking. There's not any built-in HA between the cells, at least not in the way I believe you're thinking. Each vCD cell is an active node, which provides you a management point to your vCloud environment. These cells share a back-end database, and communicate with each other as well. You can have more than 2 cells, and for larger deployments, you'll need more than 2 for the load.
Think of the vCD cells a bit like having multiple webservers in a pool, somewhat like this:
server1.www.mydomain.com
server2.www.mydomain.com
etc.
In this case, your website is www.mydomain.com. In order to serve your website from this URL, you'd put these servers behind a load balancer. If one of the back-end webservers should fail, the load balancer would drop it out of the pool, and the remaining servers would carry the load.
In vCloud, it's the same concept. You can go to the URL of each individual cell, and log in to manage the environment. However, if the cell is down, you obviously won't be able to log in, and you'll have to manually try the next cell. That's where the load balancer comes into play - it will detect the vCD cell has gone down, and direct your session to one of the remaining cells, without you having to manually try the next cell. This will, in effect, get you the high-availability you're looking for. The vCD proxy server is what allows you to get to the console of your guest VMs, and would also need to be protected in this fashion.
Now, for your original question: I believe you're asking if you can use the vShield Edge load-balancing to act as the load balancer in front of the vCD cells, instead of having to build/buy some other load balancer. Is that correct?
If so, take a look at this page:
Load Balancing vCloud Director Cells with vShield Edge | Tom Fojta's Blog
I haven't attempted this myself, so I can't guarantee it will work, but it looks promising.