Some of the comments are on using larger propellers and one 3-bladed and a 2-bladed, and there is a question on using a spreadsheet for single propellers.
- The single propeller spreadsheet may be a compilation of experience on using one single propeller. However, you will be driving two propellers.
- A main selling point for your set-up is elimination of torque. Use one 3-bladed and one 2-bladed propeller, and the difference in drag will re-introduce torque. You have 4 blades in total already, no need to add a 5th or more for any purpose.
- A larger propeller uses more horsepower than a smaller propeller, and you're driving two propellers. If your engine can drive 2 larger diameter propellers, by all means go ahead.
- Your vehicle travels at a maximum of 70 km/h, the propellers won't have an issue with attracting enough air.
- Increasing output RPM would be a good way to increase thrust!