Is the import from tumblr only a one time thing or can we re-import again in the future?
I had a look at your WordPress.com site, and it appears your import has completed.
Yes, you can import from the second Tumblr site, on the same WordPress.com site now. One just cannot perform two Tumblr imports at the same time, but can do the second when the first has been completed.
Alternatively, is there a way to ‘live’ cross post from tumblr?
When you say live cross-post, are you asking if new posts published on Tumblr can be automatically beamed to your WordPress.com site? If that’s your question, no that is not possible.
You can re-import from the same Tumblr site to bring in any new stuff posted since your last import. Previously imported content will just be skipped.