If i have a split system with funace in garage and condenser outside should i have the filter in the return up in ceiling or under blower in funace or both?

There are a few things to consider.

1. For garage installations the furnace must be a minimum of 18” off the floor to prevent the possibility of igniting flammable vapors.
2. One filter is recommended whether in the ceiling or under the furnace, that choice is up to the installer, although by the furnace that would facilitate more frequent changes.
3. The return air temperature can never be below 55 degrees, if colder than 55 ,the flue gases will condense in the heat exchanger leading to an early failure.