[Refplace]

I tend to run a mix of systems for my Fantasy campaigns.
In the world I have been working on for the past decade or two I have several so I obviously like the concept. However you have to be careful as players can get overwhelmed with choices.


For various RPM systems I decide first if they use the same power source (Mana, Spirit, Chi, Will, or Divine) to see if you need different versions of Magery.
Assuming they are the same, my next choice is are they a Magical Style or a totally different Tradition.
For a different Tradition I would simply say choose which one to roll against rather then blending them. Its simpler and less fussy.
I would not let a blending occur as you describe in most cases.
If I did I would go with Invention or Gadgeteering penalties and probably require Gadgeteering (Thaumatology) to represent an insane level of inspired genius.