OCA certification as a whole consists of two parts rightRight.
Are the above two separate certifications?No. They are separate tests. The two tests together will grant you the Oracle Database 11g Administrator Certified Associate certification.
If yes then do we have to give them at the same time or there can be gap between themThey can be taken at widely separated times. There is no time limit beyond the retirement of the exams. You might take 051 tomorrow and 052 three years from now.
Also could you please let me know the cost of the above two certifications.1Z0-051: $125 US