Cross Account EC2 Access from within Amazon Workspace
Use a VPC peering between the two accounts so that your instance from account A will be able to communicate with account B.
This will work if you have different subnet CIDR blocks in the two accounts. If you have the same (i.e. in both VPC, your subnets are in 192.168.0.0/16), then change the one that has the least impact and proceed with a VPC peering.
Finally, you will have to configure your security group to allow the bastion to communicate with the other instance.
Here are more details for AWS VPC peering: VPC peering