A simple question About this circuit

Consider 2 anti parallel Thyristor & there is a series resistor with these thyristor.Why we should consider a separate pulse of gate circuit for each thyristor

can you please rebrief your question... also in which part of the circuit is this configuration used....

Because the pulse is applied between the gate and the cathode for each thyristor.
As long you have antiparallel connection you must use at least on transformer (optocoupler etc) for one gate impulse . Once you have one impulse transformer is easiest to design it for two outputs (for driving two thyristors).
Capisci?

With one transformer with one primary winding and two secondary windings:

Use secondary windings in antiphase. Connect the begining of coil1 to Th1 gate, the end of coil1 to Th1 cathode. Connect the end of coil2 at the Th1 gate and the
beginning of coil2 at the Th1 cathode. Both thyristors must have antiparalel diodes between gate and catode and series limiting resistors in gate. Thyristors need positive pulses on gate.
Positive pulse in primary will drive Th1, negative pulse in primary will drive Th2.

With one transformer having one primary coil and one secondary coil:

Secondary of the transformer connected between Th1 gate and cathode. The same limiting resistor and antiparallel diode is must.
Th2 driven directly from the pulse circuit, gate to driver transistor (in colector or emiter depends by driver configuration), cathode to the driver ground.
Th1 is floated from the driver circuit, Th2 is galvanic coupled with the driver circuit.