Are two T1's the same as a 3Mbps pipe?

Link Aggregation
No, however by balancing two T1 links you are able to utilize the same amount of bandwidth as two bound T1 connections when multiple sessions are being utilized.
 
What does this mean?  Basically network load balancing uses session balancing in order to share connections between multiple WAN links.  Thus when a web browser opens a web site, each image is a different session, thus both links are being used to download the various files.
 
If you are using an FTP application (or something similar) it only uses a single session, and thus a single FTP download does not take advantage of multiple WAN links, however if multiple FTP sessions are started from one or more users, than both links may be utilized in order to share the load.

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