Datablending requires aggregate calculations to be utilized since we are always aggregating at the level of the data linking the two data sources. The calculation will not error if ATTR() (or another aggregate function such as MIN(), MAX() or AVG() is used with [Field1] however it is important to be mindful of the linking fields used between the data sources and how these fields are used in the view. While this is possible, I am guessing it will not provide the desired results.
Instead, what about creating a calculation in each data source such as Left([Field1],3) and Left(Field2],3) and datablending with these two new fields as linking fields? The downside is Field2 and Field1 would need to have matching data fields for a set number of characters (such as 3 in this example).
Yes that is the downside, I don't have a fixed set of characters to match.
Any other ideas?