I solved the issue. Here is the solution for everyone's reference. The trick was to use the .set_time_next_pps() and .set_start_time() functions. More details on these functions are available here. The following hand code must be added to the python file generated by the GNU Radio companion, or to another hand coded python/gnuradio program.
Take note of the lines which are added immediately after creation of the source/sink blocks, but before .set_center_freq() is sent for each which would otherwise start the streaming immediately.
# create a zero time and a start time
zero_time = uhd.time_spec_t(0.0) # hand coded
start_time = uhd.time_spec_t(2.0) # hand coded
# Add following lines following creation of source block
self.uhd_usrp_source_1.set_time_next_pps( zero_time ) # hand coded
self.uhd_usrp_source_1.set_start_time( start_time ) # hand coded
# Add following lines following creation of sink block
self.uhd_usrp_sink_0.set_time_next_pps(zero_time ) # hand coded
self.uhd_usrp_sink_0.set_start_time( start_time ) # hand coded