STS(Security Token Service)を使ってアカウント間のリソースを共有する ref: http://qiita.com/uzresk/items/dc79545721575bce5721

file0.txt

{
  "Statement": [{
    "Effect": "Allow",
    "Action": "sts:AssumeRole",
    "Resource": "arn:aws:iam::xxxxxxxxxxxxxx:role/EC2FullAccessRole"
  }]
}

file1.txt

package aws.ec2.sts;

import com.amazonaws.auth.AWSCredentialsProvider;
import com.amazonaws.auth.BasicSessionCredentials;
import com.amazonaws.auth.profile.ProfileCredentialsProvider;
import com.amazonaws.regions.Region;
import com.amazonaws.regions.Regions;
import com.amazonaws.services.ec2.AmazonEC2;
import com.amazonaws.services.ec2.AmazonEC2Client;
import com.amazonaws.services.ec2.model.DescribeTagsRequest;
import com.amazonaws.services.ec2.model.DescribeTagsResult;
import com.amazonaws.services.ec2.model.Filter;
import com.amazonaws.services.ec2.model.TagDescription;
import com.amazonaws.services.securitytoken.AWSSecurityTokenServiceClient;
import com.amazonaws.services.securitytoken.model.AssumeRoleRequest;
import com.amazonaws.services.securitytoken.model.AssumeRoleResult;

public class Ec2InstanceListUseSTS {

    /** Aのリソースにアクセスするためのロールの*/
    private static final String ARN = "arn:aws:iam::xxxxxxxxxxxx:role/EC2FullAccessRole";

    public static void main(String[] args) {

        // BアカウントのProviderを取得する。
        AWSCredentialsProvider provider = new ProfileCredentialsProvider(
                "uzr_api");

        // STSサービスを利用するためのClientを取得します。
        AWSSecurityTokenServiceClient client = new AWSSecurityTokenServiceClient(
                provider.getCredentials());
        AssumeRoleRequest request = new AssumeRoleRequest().withRoleArn(ARN)
                .withDurationSeconds(3600)
                .withRoleSessionName("EC2FullAccess_uzr_session");

        AssumeRoleResult result = client.assumeRole(request);

        // Aアカウントのリソースにアクセスするための一時的な認証情報を取得します。
        BasicSessionCredentials tempCredential = new BasicSessionCredentials(
                result.getCredentials().getAccessKeyId(), 
                result.getCredentials().getSecretAccessKey(),
                result.getCredentials().getSessionToken());

        // 一時的な認証情報を使って、EC2のインスタンス一覧のタグ名を取得します。
        AmazonEC2 ec2 = new AmazonEC2Client(tempCredential);
        ec2.setRegion(Region.getRegion(Regions.AP_NORTHEAST_1));

        DescribeTagsRequest describeTagRequest = new DescribeTagsRequest()
                .withFilters(new Filter().withName("resource-type").withValues(
                        "instance"));

        DescribeTagsResult tagsResult = ec2.describeTags(describeTagRequest);

        for (TagDescription description : tagsResult.getTags()) {
            System.out.println(description.getKey() + " : "
                    + description.getValue());
        }

    }

}