In systems consisting of multiple clusters of processors interconnected by relatively slow communication links, co-allocation may be required. We study its performance by means of simulations, depending on the structure and sizes of jobs, and the communication speed ratio. We model a multicluster with C clusters of identical processors. The workload consists of rigid jobs that require fixed numbers of processors, possibly in multiple clusters simultaneously. A job is represented by a tuple of C values, each generated from a same distribution D. In an ordered request the positions of the components in the tuple specify the clusters from which the processors must be allocated. For an unordered request, by the components of the tuple the job only specifies the numbers of processors needed in the separate clusters. A flexible request specifies the total number of processors, obtained as the sum of the values in the tuple. For total requests, there is a single cluster and a request only specifies the total number of processors needed. All intracluster communication links have the same speed, as do all intercluster links. The communication speed ratio is the ratio between the time needed to complete a send operation between processors in different clusters and in the same cluster.