I suspect you installed PP on one of the two computers awhile ago, and then another one recently?
If so, then you fell victim to MS putting the "release candidate" of PowerPivot v2 out on powerpivot.com and not making that clear.
Anyone who downloads PowerPivot recently unknowingly gets v2 which is not compatible with v1.
Is that the problem here? I have never had trouble with 32 vs 64 bit sharing a workbook.