Use the -j argument to specify the job ID of another job with which the new job should collocate.
For example, to cause job hpc-job to be collocated with a job whose job ID is 4622, enter
hpc-demo% bsub -I -n 4 -q hpc -sunhpc -j 4622 hpc-job
Use bjobs to find out the job ID of a job. See the LSF Batch User's Guide for details.