Keyboard Maestro 6 includes built in support for syncing.
First, all except the first Mac will have their macros entirely replaced, so if you have different macros on each Mac you should export any unique macros for latter import after you have set up syncing.
Macro syncing is all or nothing, so all your macros will be on all your Macs, and you can edit them on any Mac. You can set a Macro Group to be disabled specifically on this Mac by editing it in the editor.
As long as you don't make changes on two Macs faster than Dropbox syncs you will not get any conflicts, but if you do, Dropbox will save a conflict file and one or other change will be lost. You will probably need to periodically delete any conflict files Dropbox generates.