MPLS Traffic Engineering Help

Hi Everyone,
I could use a little help on MPLS and Traffic Engineering. Right now I'm
just trying to wrap my head around it. I currently have a couple tunnels
going in either direction, those are working fine (but certainly took me
long enough to get them working!) and I can direct traffic over them easy
enough. Now I'm looking into allocating/reserving bandwidth for a given
tunnel and if possible have it react to increased network loads and
recalculate its path if need be.

(Poor) Example:
I have two paths that two different tunnels (A and B) that will go over a
T-1 and 100mbps ethernet. A is more important than B. When traffic is low,
I'd like them both to go over the 100mbps link so either tunnel can fill
the pipe, but if Tunnel A requires more bandwidth, Tunnel B should react
and move to the T-1. Is this possible? or am I horribly confused?

I'm not really looking for the exact commands or the 'answer' to this
problem, but some guidance would be greatly appreciated. I'm working with
Cisco gear, 2800's and such. This is purely an academic exercise.