.. need to be able to select multiple customers-contacts per instance

Try Debra Dalgleish's sample file at:
http://www.contextures.com/excelfiles.html

Under Data Validation,
look for: DV0017 - Select Multiple Items from Dropdown List

It's a very informative sample
where several techniques are illustrated in different tabs

Since you want the "multi-selected" subjects to be ... all listed in a cell?,
viz your line:
.. to be able to select multiple customers-contacts for cell C1

then a close fit might be Debra's example in her tab:
CommaSeparated

Celebrate it, click YES below
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Max
Singapore
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