My method of merging is to use SimPE and drag all packages in that I want to merge. You can also merge using Lazy Duchess CCMerger and then edit as follows.
This is pretty much the same as merging recolors where you need to create a unique instance for each mmat. For fences we need to do this with the XFNC (xml)** resource. You can use the Hash Generator by using a different string for each fence, then use the hash value from either crc24 or crc32. Copy that value and place in the Instance box on the Resource tab.
Thanks to itsdiamondeyesuniverse for the info on changing the stringsetid as well.
Using the XFNC name create a hashed value and use the CRC 24. Place this value in the stringsetid in the XFNC (xml). This step is easier, and only works, if each fence has a unique name. If they don’t you can simple change the name in the name line.
Then select the STR for the same fence, and then on the Resource tab change the Instance value to match the hashed value from the step above.
Still trying to figure out exactly how fences are ordered in game. For cc versions, first is by price, then it appears to go in reverse alphabetical order of the package name. In the following picture there are two groups of Island fence recolors. For Maxis fences, best guess is the stringsetgroupid or resourceid after price. I have found if you use the Unique Instances button, then it reorders in game the fence. If you give instances manually it keeps the original reverse name order. <shrugs> Sort order may also be affected by which folders your fences are in. IsaWP confirmed their fences are ordered in reverse alphabetical, but not price. <shrugs>