I know a couple and they go by “Mama” and “Mummy” (mummy to me sounds the same as I imagine Mami would). They have 2 daughters. It’s never confusing as far as I can tell. Go with the names you want, and little one will pick up on it 
It is not clear to me from your post if beyond names you will be speaking to/teaching your child Urdu and Spanish, but if you are I think that will reinforce the fact that these are completely different names in completely different languages. I also suspect it would naturally encourage the division in pronunciation you are hinting at in your phonetic spelling.
That being said, like someone else mentioned depending on the particulars of your adoption I would suggest being flexible. If you happen to adopt an older child they may well have their own opinion of what they feel comfortable calling you, at least initially, for instance. Or if you adopt a child of another cultural/linguist background it might be nice to incorporate that into the names/words you use. Or if it is an open adoption perhaps you can consult with the birth mom or family.
I’ve met many families in my life involving stepparents and stepchildren. But none of them were nearly as full of love as the two mother family that my sister-in-law (my wife’s sister) and her wife have. Each of them brought a teenage daughter into the relationship. Both mothers love each other’s daughter as if she were their own; both daughters love each other’s mother as if she were their own; and both girls love each other as if they were sisters. Both daughters call their own mother “Mom”, but the other mother by her first name.
The older daughter and her wife married two years ago. Her wife is now over three months pregnant. This will be our second two mother family in our circle. Thanks to IVF, both mothers will be the baby’s biological mother. It will be interesting to see how the daughter (they already know this) will address each of her mothers.