Hi Warren, simply join the two tables together using the Join Field tool (ArcGIS Help (10.2, 10.2.1, and 10.2.2)
Use a common Id field as the join_field, and you can join whichever of your fields from the second table you need to use in calculations.
I'm assuming the model builder element labeled "this value is a constant" means that it is a one-row table, with a value you are dividing rows by in the other table.
If that's the case, you could use the Get Field Value tool to pull the value out from your one-row table into a model variable. Then you can use Calculate Field, including that model element in the expression like this:
!VFIELD! / %field value%
where "field value" is the model element output from the Get Field Value tool.
By the way, I always use the PYTHON parser (instead of the default VB parser) because it is the only one that works in both foreground and background processing.