CPU cores is like money, the more the better. Only two cores is barely enough to do the job for the webMethods stack you mentioned that includes IS, UM, and MWS. If your license agreement with SAG allows for only two cores, then maybe this is the reason why you have only two cores. What type of environment is it, DEV, TEST, QA, Production? A non-production environment may be OK with fewer system resources if money is tight. A production environment will need enough system resources to do the job.
Here is something to think about; it is a comment I hear from a SAG representative. MWS is a non-licensed product. So, consider moving MWS to an alternate server and throw lots of resources at it. Then your server with only two cores will only have to serve IS and UM.