That version of coolant is equivalent to an older G12+ VW Group spec, now when it was launched, G13 could be added to G12+, but really doing it the other way round is not smart.
Yes I understand that you have an as yet unresolved loss of engine coolant issue, so at some point the coolant in that system will get drained and disposed of, so maybe in this case, why don't you just "use up" that G12+ coolant but only as long as you then quickly get the leak solved and the entire coolant replaced.
As a disclaimer, it might just be that your cooling system has some "new" materials that discount the use of G12+ - but that is a risk that you need to evaluate.
So, to round up:- "no absolutely don't mix them" > "yes it could be okay" > "maybe" !! Sorry, you know the best plan is to buy some G12evo to top it up with out any worries, and dispose of that G12+.
