Ted has done a custom property with a couple of fields, take a look at how it works here:
And get the source for it here:
Hope that help!
I would recommend reading Allan's blog post: http://world.episerver.com/blogs/Allan-Thran/Dates/2014/6/Strategies-for-Custom-Properties-in-75/
Especially #2 about multi-selects.
I'm trying to implement a custom property that have more than 2 fields (key,value) but 3+ with no success.
any idea how to implement it? thank you