How to properly scale Jenkins?
There are no fundamental technical issues with running multiple jenkins slaves on the same machine. In fact Running Multiple Slaves on the Same Machine lists several good reasons for doing it:


  While the correct use of executors largely obviates the need for
  multiple slave instances on the same machine, there are some unique
  use cases to consider:
  
  
  You want more configurability between the configured nodes. Say you have one node set to be used as much as possible, and the other node
  to be used only when needed.
  You may have multiple Jenkins master installations building different things, and so this configuration would allow you to have
  slaves for more than one master on the same box. That's right, with
  Jenkins you really can serve two masters.
  You may wish to leverage the easiness of starting/stopping/replacing virtual machines, perhaps in conjunction with Jenkins plugins such as
  the Libvirt Slaves Plugin.
  You wish to maximize your hardware investment and utilization, at the same time minimizing operating cost (e.g. utility expenses for
  running idling slaves).
  


In general scaling out is preferred, primarily because ability to scale up is typically limited by the types/sizes of the available physical resources.

In particular for augmenting build power I'd recommend an analysis of your actual build  to determine how it uses the machine resources, which/where its bottlenecks are and what scalability limitations it raises to reveal if scaling up even helps. 

For example I encountered cases in which the build time for 2 parallel builds on the same machine was longer than the combined build times of the same 2 builds executed sequentially (non-overlapping) on the same machine. In such case I wouldn't even consider scaling up as it would actually decrease the overall building capacity.