You should check out this answer by Hans, as it's not as simple as it sounds.
Yes it can. Assuming you are hosting the application yourself, you simply need to set up two (or more) application servers pointing to the same database instance.
Images and attachments are normally stored on the Mendix server filesystem - when you have more than one Mendix server you need to configure a shared location for these files (which is configured in the Mendix Service Console).
Then you have two or more Mendix Business Servers updating the same database - all you then need to do is add a load balancer in front of the Mendix servers to distribute the load and set up appropriate DNS entries.