How to connect slicer

Try following steps:

1. add offending Slicer fields to both Pivots (temporarily)
2. right click on newly added field name, check/uncheck "Include New Items in Manual filter" (i.e. opposite of default setting)
3. click OK
{should get a dialog at this point that change will be applied to both Pivots etc}

repeat steps 2 & 3 if you want to revert to the original setting by reinstating original checkbox status (checked/unchecked)

then, finally
4. remove the fields added (Step 1) from Pivot
5. amend connections on both slicers to include 2nd Pivot

in short, the settings for the field(s) on both Pivots is different -- the above will bring the settings into line such that the slicer can be activated on both Pivots.