set_clock_group constraints question!! plz help..

You could use something like this:

set_time_format -unit ns -decimal_places 3
# Create the first input clock to the mux
create_clock -period 10 -name clockA [get_ports a_clock]
# Create the second input clock to the mux
create_clock -period 10 -name clockB [get_ports b_clock]
#Cut transfers between FirstClock and SecondClock
set_clock_groups -exclusive -group {clockA} -group {clockB}

or the following for externally switched clocks

set_time_format -unit ns -decimal_places 3
# Create the first input clock on the clock port
create_clock -period 10 -name clockA [get_ports  a_clock]
# Create the second input clock on the same clock port
create_clock -period 10 -name clockB [get_ports b_clock] -add
# Cut transfers between FirstClock and SecondClock
set_clock_groups -exclusive -group { clockA } -group { clockB }