(If anyone's reading this who hasn't already seen the previous question, I recommend you read that first so that this answer makes sense in context.)
There is no "best" Shiny for use in breeding. In Gen II breeding, any Shiny is as good as any other. All you need to do is ensure that the two Pokemon are compatible, ie. fit within the guidelines outlined in the answer to your previous question.
In fact, I'm not entirely sure that using two Shiny parents instead of one will increase your chances of getting a Shiny baby at all. Perhaps it will increase the chance of a Shiny-compatible IV combination being passed down since both parents now posses a lucky combination. As I'm no expert with all the numbers behind breeding mechanics (especially in Gen II), I don't know for sure.
Overall, I recommend that you simply change your Umbreon's breeding partner to anything it's compatible with and see if you get lucky with the baby Eevee. (Note, if your Umbreon is a male, you'll have to breed it with a Ditto to get a baby Eevee, as only the female will pass the species down in any other situation.)