Two walls with the same hatch as one wall should present the same performance hit as one wall.
Note also that MaxHatch is case-sensitive and must be entered as shown. The "number" you set it to must also be a quoted string. If you find that as tedious as I did, you may find the AutoLISP routine in the attached ZIP file of some use, as, when loaded, it requires only typing "maxh" in any combination of upper and lower case letters as you see fit to use. You will be prompted to enter a number and the routine takes care of the formatting. If you enter a number below the minimum value, the routine will substitue the minimum value [100]; likewise, an number over the maximum value [10000000] will result in the maximum value being used.
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David Koch
Autodesk Discussion Group Facilitator
Using ADT 2004 at work; access to 2005, 2006 & 2007 at home