Q: Do they ever allow 2 non-sibling placements @ same time?

Depends on where you're at. In my county, it wouldn't matter particularly if I were adopting children through foster care. If two of my placements went toward adoption, it isn't as though they would remove one or halt one of the adoptions. Some places have a rule or guideline that they prefer a family to wait a year between adoptions. I understand why they would want that, but the reality is that there aren't enough foster homes and no one controls how the cases work out.

My kids are not blood related, they are 4 month apart, both placed as newborns, both were looked at as pre adoptive placements, due to history of bios.

They have no problem with it at all, where I am. I know several families who have done it.

Here, if they are placing a child as a regular foster placement, it doesn't matter if you have another regular foster child already there. and if both cases turn toward adoption at the same time, that's fine too. But if they are placing a child with an adoption plan, they will not place that child in a home that already has a child with an adoption plan. If you already have a child with an adoption plan in your home, then sometimes they halt placements entirely, and sometimes they halt placements of a certain type. It depends on the family's worker and their beliefs about what the child with the adoption plan needs.