To connect to multiple AWS accounts using different IAM users, following are the AWS CLI configuration steps.

$ aws configure --profile
AWS Access Key ID [None]:
AWS Secret Access Key [None]:
Default region name [None]: <region, default is us-east-1>
Default output format [None]: <text, json or table; default is json>

This would add user2 in the config file.

$ more ~/.aws/config
region = us-east-1
output = JSON
[profile user2]
region = us-east-1

Note that the credentials are stored in the credentials file.

$ more ~/.aws/credentials
aws_access_key_id = ***
aws_secret_access_key = ***
aws_secret_access_key = ***
aws_access_key_id = ***

To connect to the first AWS account using the default profile:

$ aws s3 ls
2017-03-02 07:23:57 bucket1
2017-03-02 07:37:16 bucket2
2017-03-02 07:39:17 bucket3

To connect to the second AWS account using user2 profile:

$ aws s3 ls --profile user2
2017-03-04 09:43:03 bucketA
2017-03-04 10:11:18 bucketB

More detailed instruction here.