Try this:

Invoke Replace dialog (default key: ctrl+h)
Find what zone: (?-s)^(.+)\R(.+)(?:\R|\z)
Replace with zone: \1:\2
Wrap around checkbox: ticked or unticked, as you like
Search mode selection: Regular expression
Action: Press Replace or Replace All button, as you like

Here’s how it works:

THE FIND EXPRESSION:

(?-s)^(.+)\R(.+)(?:\R|\z)

• [Use these options for the whole regular expression][1 ] (?-s)
  • [(hyphen inverts the meaning of the letters that follow)][1 ] -
  • [Dot doesn’t match line breaks][1 ] s
• [Assert position at the beginning of a line (at beginning of the string or after a line break character) (carriage return and line feed, form feed)][2 ] ^
• [Match the regex below and capture its match into backreference number 1][3 ] (.+)
  • [Match any single character that is NOT a line break character (line feed, carriage return, form feed)][4 ] .+
    • [Between one and unlimited times, as many times as possible, giving back as needed (greedy)][5 ] +
• [Match a line break (carriage return and line feed pair, sole line feed, sole carriage return, vertical tab, form feed)][6 ] \R
• [Match the regex below and capture its match into backreference number 2][3 ] (.+)
  • [Match any single character that is NOT a line break character (line feed, carriage return, form feed)][4 ] .+
    • [Between one and unlimited times, as many times as possible, giving back as needed (greedy)][5 ] +
• [Match the regular expression below][3 ] (?:\R|\z)
  • [Match this alternative (attempting the next alternative only if this one fails)][7 ] \R
    • [Match a line break (carriage return and line feed pair, sole line feed, sole carriage return, vertical tab, form feed)][6 ] \R
  • [Or match this alternative (the entire group fails if this one fails to match)][7 ] \z
    • [Assert position at the very end of the string][8 ] \z

THE REPLACE EXPRESSION:

\1:\2

• [Insert the text that was last matched by capturing group number 1][9 ] \1
• [Insert a colon][10 ] :
• [Insert the text that was last matched by capturing group number 2][9 ] \2

Created with RegexBuddy

[1 ]: https://www.regular-expressions.info/modifiers.html
[2 ]: https://www.regular-expressions.info/anchors.html
[3 ]: https://www.regular-expressions.info/brackets.html
[4 ]: https://www.regular-expressions.info/dot.html
[5 ]: https://www.regular-expressions.info/repeat.html
[6 ]: https://www.regular-expressions.info/nonprint.html
[7 ]: https://www.regular-expressions.info/alternation.html
[8 ]: https://www.regular-expressions.info/anchors.html#az
[9 ]: https://www.regular-expressions.info/replacebackref.html
[10 ]: https://www.regular-expressions.info/characters.html

RegexBuddy settings to emulate N++ regex engine: Application=boost::regex 1.54-1.57 / flavor=Default flavor / replacement flavor=All flavor / ^$ match at line breaks / Numbered capture / Allow zero-length matches