With GNU awk for multi-char RS and \s:
$ awk -v RS='\\s+' '{ORS=(NR%2 ? OFS : "\n")} 1' file ABC 1223334 Days 344678544324677 Base 45666 ABC 1234565 Days 234567899765443 Base 456643 Ed Morton
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With GNU awk for multi-char RS and \s:
$ awk -v RS='\\s+' '{ORS=(NR%2 ? OFS : "\n")} 1' file ABC 1223334 Days 344678544324677 Base 45666 ABC 1234565 Days 234567899765443 Base 456643