Thanks for the help, I appreciate it. There's a lot more to it than throwing blades and beads together and fishing, huh?
I dug up some old baits and components and tried to do some testing. Using a blade sizing chart of the web, I found two #5 Indiana's and one #6, and also some #6 Colorado's. I put the two 5's on a 3/8oz bait and tested it in the pool. At all but the fastest speed, the rear blade just flopped about and the front blade turned nicely--and bait tracked perfectly. Modest vibration. I put the #6 colorado on the back, and the bait ran great, and vibration (thumping) was very good. I then put the #6 Indiana on the back, and the bait vibrated like a jackhammer and tracked perfect at all speeds. Awesome feel.
I think I will try what you said with a smaller Colorado in front of the 5 Indiana too. Based on what I've seen today---if the chart I used is right, for a tandem Indiana on a 3/8oz head, a 4-5 or a 5-6 combo makes a heck of a bait and displaces water really well.
I'm working toward putting an order into lurepartsonline.com tonight. Thanks again for you insights! I attached a pic of the 3/8oz bait with the 5 and 6 Indiana's.