I doubt it is possible since Apache is most likely not listening for a response back from the default DocumentRoot?
The other option, if possible, is to have Heartbeat change the Mount point on the Apache machine if the machine serving up mnt1 (.103) is unavailable? (Heartbeat drop the mount to .103 and mount backup NFS2 (.104))
Apache has no directory failover functionality built in. You need something on the system level to change mounted disks whenever something happens with the connection to the original mount. I have no experience with Heartbeat or other utilities to do this, so can't advice you on that. Maybe another member can jump in and help you.