I do not think you can run the same engine with the fuels you mentioned. Hydrogen need to be ignited after compression by a spark and here you need a spark ignition engine. Biodiesel can self ignite when compressed to higher pressure and a compression ignition engine is suitable for that. The two engines fuel supply and ignition systems are completely different unless you do some mixing between these two fuels.

Less environmental pollution

better performance of vehicles

Maciej I agree with what you said but how do you achieve the mixing process of the two fuel and  what are the suitable ratios. Then how are the two fuels going to be produced and stored near the test facility. As you said so many challenges need to be resolved.

I agree with Maciej, There have already been attempts to use premixed LPG-air which was used as charge inside the cylinder, compressed and then diesel was conventionally sprayed at the end of the compression stroke.

The high temperature of the charge (at the end of compression) would first initiate the combustion of diesel atomized droplets, consequent heat release would provide the ignition energy for the LPG molecules which would eventually combust. Care is to be taken to prevent the LPG from auto-igniting before the spray of the diesel.

In this case, like Hydrogen, LPG is a SI engine fuel, but was used as a secondary fuel to diesel.

For Hydrogen as the secondary fuel in diesel engine:

Would suggest theoretical studies using thermodynamics for the rise in temp of H2-Air mixture, sequential thermo-chemical equations, one for the H2 reaction with O2 and second for Diesel HC's with O2 employing the blend ratio as an averaging index in order to calculate the power output of the dual fuel combustion, heat release rates.

u need to match the fuel viscosity and density and flash point to diesel by doing transesterification.... then only you can get good results compared to directly using the oils..