I used to get asked how to combine two sets all the time when my repair shop was open. To save myself some time, I drew up some basic plans using RR-Track. Here is a basic double loop using two set loops, all 18" radius curves, two left hand switches, two right hand switches, and four 2" straight sections. It works with EZ-Track or Tru-Track:

You'll need a second power supply, and insulated rail joiners between the switches that make up the crossovers, if you plan to operate two trains independently on each loop. Or upgrade to DCC, and locos with decoders in them, to allow operation of multiple locos without doing that.
Also note the 'S' curves going through the crossovers, they can be a problem with long cars. So stick with small locos, and 50' or shorter cars, and you should be fine. You'll want locos around the size of F7's or GP9's, or smaller. Forget big steam and modern "SD-MAC" size diesels.
Edit to add: Note the outer loop is offset slightly from the inner. This is a result of using 'standard' switches while keeping everything within a 4'x8' layout.
Len