This is how I manage multiple AWS credentials.

gistfile1.sh

# Author: João Abecasis <joao@comoyo.com>
# Date:   2013-06-19
#
# For each AWS identity, you'll need
#
#   ~/.aws-credentials.<identity>   -- with AWS credentials (see below)
#   ~/.aws.<identity>               -- symlink to this script
#
# The file .aws-credentials.<identity> is the canonical source for credentials.
# It's directly used by some AWS tools and it should look like:
#
#   AWSAccessKeyId=XXXXXXXXXXXXXXXXXXXX
#   AWSSecretKey=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
#
# In bash, load credentials with:
#
#   $ source ~/.aws.<identity>
#

aws_identity=$(basename "${BASH_SOURCE[0]}")
aws_identity=${aws_identity#.aws.}

echo "Loading AWS identity: $aws_identity"

# For AutoScaling tools
AWS_CREDENTIAL_FILE=~/.aws-credentials.${aws_identity}
declare $(cat ${AWS_CREDENTIAL_FILE})

# For EC2
AWS_ACCESS_KEY=$AWSAccessKeyId
AWS_SECRET_KEY=$AWSSecretKey

# Default region
AWS_DEFAULT_REGION=eu-west-1
EC2_REGION=${AWS_DEFAULT_REGION}

# Service URLs, for services that don't support EC2_REGION
EC2_URL=https://ec2.${EC2_REGION}.amazonaws.com

export AWS_CREDENTIAL_FILE AWS_ACCESS_KEY AWS_SECRET_KEY AWS_DEFAULT_REGION \
    EC2_REGION EC2_URL