Netbackup DD to DD replication

This really is a question for Dell/EMC as replication throttling is managed by the DD, not NetBackup (NetBackup merely tells the DD to replicate - the DD's manage the data transfer between themselves). 

That said, it looks like the answer depends on how you are configuring throttling - if you are only setting it at the source, then I think you will see 2 mbps in-bound and 1mbps out-bound on DD3 during business hours (and 20 in and 10 out during off business hours). 
See this thread, which I thnk may help you to understand how it worsk: https://www.dell.com/community/Data-Domain/Throttle-the-bandwidth-of-replication-possible/td-p/7169087
From reading the article it appears you are able to set throttling on either source or target of the replication.

HTH