Replace your simple Redirect directives with mod_rewrite rules that check the hostname:

# Redirects for myrnainterior.com

RewriteCond %{HTTP_HOST} ^(www\.)?myrnainterior\.com$ [NC]

RewriteRule ^interieur-styling$ /interieur/interieurstyling [L,R=301]

RewriteRule ^interieurstyling$ /interieur/interieurstyling [L,R=301]

RewriteRule ^renovatie$ /verkoopstyling [L,R=301]

RewriteRule ^fotos-interieurstyling$ /projecten [L,R=301]

RewriteRule ^fotos-verkoopstyling$ /projecten [L,R=301]

RewriteRule ^fotos-renovatie$ /projecten [L,R=301]

RewriteRule ^cookiebeleid$ /privacy [L,R=301]

# Redirects for hetwapenvanrhoon.nl

RewriteCond %{HTTP_HOST} ^(www\.)?hetwapenvanrhoon\.nl$ [NC]

RewriteRule ^brasserie/menukaart/lunch$ /brasserie/menukaart/lunchgerechten [L,R=301]

RewriteRule ^brasserie/menukaart/lunchspecials$ /brasserie/menukaart/specials [L,R=301]

RewriteRule ^hotel-historie$ /historie [L,R=301]

RewriteRule ^terras-rhn42$ /terras [L,R=301]

RewriteRule ^gasterhoon-groep$ /contact [L,R=301]

RewriteRule ^menukaart$ /brasserie/menukaart [L,R=301]

RewriteRule ^ek-voetbal$ /eetcafe [L,R=301]

RewriteRule ^ek-voetbal/voetbalpoule$ /eetcafe [L,R=301]

RewriteRule ^ek-voetbal/reserveren$ /reserveren [L,R=301]

RewriteRule ^category/events$ /evenementen [L,R=301]