I doubt your nandrolone is water based no ester, that's rare as hen's teeth. And just supposing it was, doubt you'd be able to achieve 200 mg/ml.
No problem mixing the two together, assuming they are both oil based, to cut your shots in half.
It is probably fine too to mix oil and water based, I've done it before with HCG and it makes a bit of a mess in the syringe but it all ends up inside you. I choose not to do it, so don't take this as me endorsing it - just that I have done it in the past and it seemed to work ok. My general practice is water based subQ and oil based IM.
At any rate, try it for a shot and you'll find out right away if your nadrolone is water based or not.