If you want to do this, I would suggest getting the private single, private multi, then the instrument rating in the twin. This would allow you to log PIC time during instrument training.
If you do your instrument rating in a twin, you also have instrument privledges in the single. If you do your instrument in the single, you have to do a single engine approach in the twin to get instrument privledges in the multi.
If you already have an instrument rating in the single, and add a multi rating, it is my understanding that the FAA is now requiring you to prove instrument competency in the twin, but I have not seen this in writing anywhere.