Alternate solution - just copy the **STRING value**
Consider the possibility that the users in the ''other'' tool really do not need to change the value, but might just need to know what it is.
If the bi-directional field mapping is not really necessary (often it really is not), you can just create a STRING field in the ''other'' tool to hold the current value of what is selected from the long list field.
So if we have a QC list field, say called ''MyList'' with 500 items and in Endpoint 2 we do not NEED to set the value but only know what it is, we can create a STRING field called ''QC-MyList'', for example.
We can then create a one-way QC-EP2 map from MyList to QC-MyList.
During Sync, it will just populate the QC-MyList field with the string equivalent of what was selected in QC for MyList.
This technique also works well for USER fields when you do not want to (or can't) make the user list the same or have too many users (more than 50).