With no load, Vref2, R3 and R4 cause an adjustable positive voltage on the non-inverting input of AO2. The output of OA2 will be at the full supply voltage, 33V. D2 will be reverse biased having no control of T1.
If load current is increased to the point that causes OA2's non-inverting input to drop slightly below its inverting input. OA2's output will swing low, first causing D2 to forward bias, taking control of T1 from OA1. OA2 might need to continue to pull the Base of T1 lower until the output current settles to a value that causes OA2's inputs to be very close to equal, 0V.
The purpose of D1 and D2 is to cause whichever opamp that has the lowest output voltage to take control of T1. They can be called "ORing" diodes, although not strictly correct.
BTW, it's actually the current from the Constant Current source that supplies drive current to T1.