In the case of the clocks being synchronous, you can define one timing clock on both clock source objects if originally both clocks have the same waveform (see the first example below).
Example 1: create_clock -period 10 -name clk1 [get_ports <clock-1-source> <clock-2-source>]
If the two clocks have different waveforms, you can define the first clock as a primary clock and the second clock as a generated clock, with the first clock specified as the master clock (see Example 2 below).
Example 2: create_clock -period 10 -name clk1 [get_ports <clock-1-source>]
If the clocks are related, but have a clock period ratio of 2, the solution is to create a primary clock on the one source, and create a generated clock on the second source:
create_generated_clock -source [get_ports <clock-1-source>] -name clk2 -divide_by 2 [get_ports <clock-2-source>]