Thanks guys.
Here is my write up of how I plan to proceed.
And so you don't have to follow that link, here is the redirect code I'm going to use (the 302 will become a 301):
RewriteEngine on
RewriteCond %{HTTP_HOST} ^([^.:]+\.)*testsyte1\.com\.?(:[0-9]*)?$ [NC]
RewriteRule ^(.*)$ http://one.testyng.com/$1 [R=302,L]
RewriteCond %{HTTP_HOST} ^([^.:]+\.)*testsyte2\.com\.?(:[0-9]*)?$ [NC]
RewriteRule ^(.*)$ http://two.testyng.com/$1 [R=302,L]
It works very well on my test sites. The live sites will be a bit more complicated since most of the content is in 3rd party tables and needs to be converted to a new custom post type.
But that's for another day.