Hi Manix,
If I understand you correctly, you don't actually need to use a calculated field to concatenate the two columns; when you define the combobox on your form, you simply specify the relationship and then instead of choosing one field, you enter {field1} {field2} etc etc. And you can add any text you want before, in the middle or after your fields. Please see the attached for some examples.
With regard to aligning the combobox fields, I'm sure there is a clever way of doing it but I do something more simple. I add another field to my lookup table where my tax rates are held and align everything there. So when you come to use the combobox on your form, this is the field that the user sees and it looks as you want it. Again, this is in the attachment.
Hope this helps,
Derek.