Would it work if I put check valves where the nitrogen and co2 blend/tee together, so one tank wouldn't overpower the other due to pressure?
Check valves won't solve this one.
Regulators open up when their release pressure is reached and the one set to the highest pressure will open up first, leaving the other one closed. Even if both are set to the exact same pressure, one will always be slightly lower than the other.
So, let's say your nitrogen regulator is set to 12 PSI and your CO2 regulator is set to 10 PSI? As soon as your pressure drops below 12 PSI the nitrogen regulator will open. The pressure will NEVER drop below 12 PSI and the CO2 regulator will never open.
There is a way to do it but it requires 3 regulators, 2 needle valves and 2 flow meters to measure & set the proper gas flow.
The first two regulators regulate the tank pressures to a constant 100 PSI & they do not have to be the same.
Then the needle valves are used to create a constant flow rate from the 100 PSI.
Finally the third regulator will regulate the gas down to the pressure you need.
To adjust you would set the two needle valves while delivering gas through the third regulator and setting the flow rates to 3.5 CFM and 6.5 CFM would give you a 35/65 mix.
Also, the combined flow rate of both needle valves has to be at or less than the flow rate of the third regulator. If this is not done then the mixture will be off.