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If you don't mind other chains of spaces within the lines being converted to blanks along with the newlines:

$ awk -v RS= '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

or if you want tabs, etc. retained and only newlines converted to blanks:

$ awk -v RS= -F'\n' '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

$ awk -v RS= '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

$ awk -v RS= -F'\n' '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

If you don't mind other chains of spaces within the lines being converted to blanks along with the newlines:

$ awk -v RS= '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

or if you want tabs, etc. retained and only newlines converted to blanks:

$ awk -v RS= -F'\n' '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

If you don't really have trailing blanks at the end of some lines of input:

$ awk -v RS= -F'\n' '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2

If you can have trailing blanks on input lines then:

awk -v RS= -F' *\n' '{$1=$1}1' file

$ awk -v RS= -F'\n' '{$1=$1}1' file
These are the words for sentence 1
these are the words for sentence 2