We assume that the input model is a single Stokes I plane, and that the object to be simulated in CASA is an unpolarized continuum source.
The bandwidth of the observations can be set using the "inwidth" parameter of the task simobserve. Set this parameter to the effective bandwidth of the observations you intend to carry out, defined ALMA Technical Handbook. The ALMA Technical Handbook can be accessed via the "Documentation" page of the ALMA Science Portal.
Simobserve will put both polarizations (XX and YY) into the measurement set it generates and the resultant image by simanalyze will have both polarizations coadded.