The MongoDB Production Notes has a section entitled 'Allocate Sufficient RAM and CPU' which talks about the considerations for deploying mongod nodes using either the MMAPv1 or WiredTiger storage engine.
There is a small paragraph in the WiredTiger section which talks about the considerations when running multiple WiredTiger instances on a single host, but there are no such recommendations for MMAPv1. Nor are there any recommendations around running both an MMAPv1 based mongod process and a WiredTiger based mongod process on the same host.
It would be beneficial in both cases to suggest that these are not recommended configurations, i.e. it is not recommended to run multiple MMAPv1 based mongod processes on a single host and you should definitely not run a mixed environment (MMAPv1 based mongod processes and WiredTiger based mongod processes) on a single host unless there are explicit controls in place (e.g. cgroups, containers and/or virtual machines to ensure complete resource isolation).