Can we have two primaries for the same shard in mongodb?
MongoDB does not currently support a multi-master setup such as you describe. 

If you were looking to provide localized regional writes via MongoDB, your best bet is to setup a sharded collection, using shard tag ranges. You would have 1 (or more) shard per region, with the primary residing within the region it manages and remote secondaries. Region would be the first member in your shard key and you would then peg a region's range to the shard that owns that region.

Reads for that region's data performed from a different region could then use a read preference of "nearest", which would result in a secondary read to the local secondary.