You cannot merge the 2 libraries so i would suggest that you back up the songs from the new library, then using these instructions, transfer the old library onto the new laptop and then import the new songs into that library. Here are the instructions I was given by a techy friend when i got a new laptop and it worked.

How to transfer an itunes library onto another computer

• Open itunes on old laptop with ipod connected and enable Disk Use and apply.
• In my computer, open the Ipod USB and create a folder called Itunes backup
• Go into My docs/my music and copy the Itunes music folder and itunes music library.xml and itunes music library.itl to the Itunes back up folder
• You will now need to eject the ipod manually
• On the new computer – plug in the ipod and launch itunes but tell it not to replace the library!
• Copy the contents of the Itunes back up folder from the Ipod onto the Desktop.
• Close out of itunes and eject the ipod.
• Copy the files from the desktop into the itunes folder in My Music – this will replace an old library that is there.
• Open itunes and you may then need to import the library into it if it does not do so automatically
• Attach the ipod and this time, let it wipe the ipod and replace it with the new library.  Everything will be updated except playlists.
• Go back and delete the back up folder from the ipod as this takes up memory.  You can then also uncheck the enable disk use option.

I hope this works