This is caused by all the nodes in the cluster attempting to append to the file. The fix is to limit the destination to only send the messages (append the file) on one node in the cluster. Here are the steps to configure this.
1. Configure the channel to persistence level 2:
a. Right click on the channel and go to properties.
b. slide the Persistence to 2
2. On the destination node set check the box to only process on one HA Instance:
From here just save and restart your channel and you should not see any more errors about the file in use by another process.