aws ec2 elastic ip to multiple instance

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http://serverfault.com – i am new to ec2, just finished setting up my micro instance and ready for production. i want to start w/ 2 medium instances. How do i assign my elastic IP to both of them so they act as 1? also would they share dB as well? my visualization is like a pair of raid drives where you have multiple machines acting as 1 am i going about it the right way? i have read that you should have 1 more instance than you need for redundancy. (HowTos)