Littermates

do you have a dog house on the lot to direct them to go to the doghouse and woohoo in ?

I do have a dog house and actually to ensure that I didn't lose the dog I really wanted, I used a cheat and went into Edit Sim, saved the dog to the bin, and then as you said, I merged him into my household as a new dog with zero relation so it should work. The dogs aren't best friends yet and there isn't a romantic option still. Do they have be best friends until the option is available?

I think so but friends could work as well - it does with horses. 

If you wanted to save time, you could just max the relationship anyway with testingcheatsenabled - which is, I assume, the cheat you used previously

what  andro said build up the relationship ( just like sims the relationship has to be a certain level to woohoo/try for babies ) don't forget to every so often while building up the relationship to click on the dog house once the try for shows up if succesful you should hear the lullaby chime

My pets are littermates even though i put one in different houses

and then merged them