The one will overwrite the other. They won't actually conflict and cause problems, but you will lose the sounds from whichever was first. What you can do is use your computer's Sound Recorder or a sound device such as
www.goldwave.com to listen to each and see which you like best. If you install DBSM but like Thomsen's Creaks better you can re-install the individual files on top of the other ones. You can do this by inserting them yourself or you can make a JSGME-compatible folder by putting the creaks into a folder and that one into another until you have something like 'Thomsen's Creaks/Data/Sound'. Then you can use JSGME to install that last.