If their voltages are similar, then each charger will contribute to the total amps going to the battery. However, the total amps cannot exceed the battery acceptance rate. You can have more chargers contributing than is needed, when fewer chargers could do it all anyway.
You could easily find that the alternator could do it all whether the solar is "on" or not. If it could and the solar is "on" and they are at the same charger voltage, then the solar would now contribute some of the total and the alternator would contribute less than it could.