My team is trying to figure out if using 2 cims with the normal gearbox we get in KOP would be better than using a gearbox with a 20:1 gear ratios and 1 cim. We don’t want to buy a new gearbox but I would like to know which one would be better to lift our robot.
Hard to say with the information you gave.
You should make friends with JVN’s Calc. It is wonderful.
You can input your motor choice (and how many), drive pulley/sprocket/drum size, weights, etc. and then find out how fast you would climb and how many amps you would draw.
For your application, I think you would want the “Linear” tab of the spreadsheet.
Came here to say this. We used it to determine the ratios on the Team Cockamamie lifter (video finished soon, I promise!), and we came up with 144:1 as our ratio to lift ourselves and two other robots given our pulley size and motor options. That turned out to be conservative once everything was built, but sometimes conservative is ideal for V1.
If you’ve determined that one CIM at 20:1 would be adequate but have a 10:1 gearbox, you could then gear that output down 2:1 using chain or belt. Just use a sprocket/pulley with twice as many teeth on the load side as on the gearbox side.
Edit:
Note that drum size will greatly affect the gear ratio you need. If they’re planning a 1/2" shaft for a drum and you’re planning one 3.5" in diameter, this could explain such a difference. They may also only be looking at raising themselves, or themselves and one other robot (relying on levitate).
Major keys here. (And hopefully OP already noticed them perusing the JVN spreadsheet.)
For the record, we designed around a 3-4" drum (to account for cable wrap, which was about right in practice) and a 500-pound load (which is probably about 40-50 pounds safe, and can obviously be eased up if your own robot comes in light). We also wanted to use one CIM motor run through the 36:1 CIM Sport that AndyMark sponsored, and we had a selection of sprockets to choose from. So we chose 15:60 on the sprocket to get the rest of the way. 
Need to more carefully define “better”
The quality of the solution is directly correlated to the quality of the specification.