Hey Jason,
Are the domains you register brand new? Because if they are, it wouldn't make a difference if you redirect them to the second domain or not. In this case, they wouldn't have any back links to get the link juice from.
The only reason for registering a new domain would be from the strategic perspective. You would prevent the competition to register it sooner and also you might want to run a new business on those domains. Regarding SEO, it wouldn't make a difference.
Anyway, if you want to rank high for your clients, I'd recommend to read this thread from Q&A.
Hope it helps. Cheers, Martin