You cannot have both a 32-bit and a 64-bit installation of Office in the same Windows environment.

Have you tried repairing the Office installation (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair)?

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I would have disagreed with you until today, as my older laptop let me hold both.
However even this has now started to playup, with conflicts,etc.
I suspect it has something to do with windows 10 updates, as this is a relatively new phenomena.
Now reverted to 1 laptop with office 2010 (had to remove 1 of my addins) and 1 laptop with 2013.
Still can not understand how excel 2010 lets me load all the workbook/sheets I have always worked with (taskmanager count usually shows 48) and yet the newer 2013 keeps giving out of memory problems.

You can - you just have to install the 64bit version(s) first. It's not a supported scenario as far as I know, but it does work.