It seems that some sdc solutions (like Quartus and Vivado) support the optional -add argument to create_clock, see below;

create_clock -add -name clk100mhz_i -period 10.00 -waveform {0 5} [get_ports {clk100mhz_i}];

https://www.intel.com/content/www/us/en/programmable/quartushelp/13.0/mergedProjects/tafs/tafs/tcl_pkg_sdc_ver_1.5_cmd_create_clock.htm

create_clock [-h | -help] [-long_help] [-add] [-name <clock_name>] -period <value> [-waveform <edge_list>] [<targets>]

...

   -add       Adds clock to a node with an existing clock

...

The semantics are described as;

If a clock with the same name is already assigned to a given target, the create_clock command will overwrite the existing clock. If a clock with a different name exists on the given target, the create_clock command will be ignored unless the -add option is used. The -add option can be used to assign multiple clocks to a pin or port.

We should make sure we have tests for;

• Using create_clock with the same name on a given pin/port replaces the existing clock.
• Using create_clock with a different name on given pin/port the result is ignored (but we should issue a highly visible warning that this is occuring).
• Using create_clock with -add is able to assign multiple clocks to a given pin/port.

It is likely with the current semantics people do things like;

• Step 1 - Select all pins and set constraint A
• Step 2 - Select some pins and override the existing constraint A with a different constraint B