Need info RE: electrical system on a 1995 Chevy 2500 Silverado pickup w/ 450 gas engine and dual batteries?

I am so hoping someone can help me!

1) why two batteries?

2) there is a knife switch to turn off the secondary battery, however, when turned off, truck doesn't run, is this the usual? explain?

3) Primary battery is good. Secondary battery is not holding a charge ... had it checked, was told they thought it might not be dead, could be electrical problem ... had electrical system diagnosed at a shop, they said it was fine. However, at time the power meter drops, and the lights, etc dim significantly when it does. Any ideas?

4) May be unrelated, but had to have starter replaced in July.

Thanks for any ideas.