I wonder if this would be a good combo. I like to mix 3 qt of 0W-30 German with 2 qt of CTX 5W-30. and change it at 5000 miles. Here I am lucky to find 6 qt of 03W-30 and I have 2 cases of 5W-30 GTX and 3 cars to spilt the oil between.
I think the mix would definitely be inferior to straight GC as you "thin" it out, have less TBN, etc. Why not use the straight GC in the most important vehicle or the one that likes a thicker oil?
It won't hurt, but you might be mixing a brew that is inferior to GC or GTX alone.
Adding GC to GTX might not necessarilly improve it either. This is not how Castrol semi-syns are formulated. You might thicken it a bit but dilute the different anti-wear agents in both.