Hi Gaurav,
You can only sync one Salesforce account with Marketo.Think of the word account as ORG.
You
CANNOT sync one Marketo instance to your SFDC sandbox and then switch it to sync to your SDFC Production org. Once you sync your Marketo instance to a sandbox you are tied down to that sandbox. If you refresh the SFDC sandbox then support can assist you in getting the sync up and running between the existing Marketo instance and the fresh sandbox.
Really if you want to sync your SFDC sandbox to a marketo instance then you should contact your account executive and discuss the possibility of getting a sandbox.
Your Marketo Production instance should be reserved for your SFDC production.
Rgs,
Cathal