New Table as Parent or Child or Both?

Build Table has records that must capture a number of maps used in that Build.
Maps Table has fields Map Name, URL Link, File Upload file attachment field.
On the Build Table, I want to be able to multi-SELECT the Maps and then based on this selection, ALSO bring over the URL Links for the maps selected. 
I tried setting Build as Parent and then as Child and then as Both with the Maps Table, but nothing seemed to work. 
Any ideas?

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Ruby Kapil
rubykapil@wi-finity.com
Wi-Finity, Inc
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