Is it possible to have a shared secondary Mongo DB instance between two replica sets..? I have two separate mongo db instances say A and B running in a remote location and i want to maintain a collective synchronized copy/backup of these instances in a separate mongo instance say C. So i just want to know is it possible to configure two separate replica sets for A and B instances having C as common secondary instance for both the sets..?
No you cannot have shared secondary as the sync is based on oplog replay plus a single monogd cannot be a member of 2 replica set.

The only way to have a single machine be a member of multiple replica sets is to run multiple mongod servers on that machine.