I'm new to the tactical gear scene but I like running thigh rigs. For my next carbine class, I'm running a 3 mag pouch thigh rig on my left and a pistol holster thigh rig on my right.

I loaded up with this setup last night and it seems to pull on my waist belt a bit too much.  I have a Wilderness 1-3/4" trainer's belt.

Is it normal to attach thigh rigs to the waist belt of a load bearing harness?  It seems like a fancy version of suspenders, but I'm curious if this is something reasonable to attempt?

Sounds reasonable, though I've never tried it.  I do, however, use a set up similar to yours for walking in the hills; a drop leg on the right for a pistol and a four M16 mag drop leg on the left.  While it does pull on the pants some, I have never thought it was that big of a deal if you have the leg straps secure, even when running.

<---Fancy way of tagging for more answers.

ETA:  I use a BlackHawks Duty Belt, basically a military style pistol belt to attach my leg rigs to.  I wear it on top of my normal belt.  It works better than using your normal pants belt alone, and its quicker too.

Use this with your Wilderness belt:

www.highspeedgearinc.com/proddetail.asp?prod=HSG-SUSP

Thigh rigs are most commonly used for first line gear that stays on your body no matter what--mini survival kit, compass, leatherman tool, small amount of backup ammo
(1-2 AR mags, 1 pistol mag, eg.) , and/or a drop leg holster for a pistol.  

You would not want to attach a thigh rig to second line gear like a battle vest.  

Use suspenders with your wilderness belt to hold it all up.  

The HSGI's are nice.  

Like most here, I keep my rig on a separate duty belt (uncle mike's, which I will replace with an Eagle some time).  2xAR mags balance out the pistol quite nicely.

I have my subloads attached to a pistol belt, which in turn is attached to my TT Tac vest.  That way, I don't need to have the pistol belt real tight.
I'm not a soldier, w/ the need for very distinct 1st, 2nd, etc. line gear.
If you feel that you're needs for the class are addressed by your current set up (ammo wise), I'd seriously consider the HSGI suspenders or equivalent for comfort.
Check w/ your class coming up.  If 3 mags are enough realistically, then you're getting there.
Some classes mention bare minimums, but when you get there, you find that if everyone had a real good ammo supply, water, snacks (watch for lead on your hands), band aids, etc that you don't have as much down time.  Then you get more trigger time, which is always a good thing.