What Pontius wrote. You will get the same exposure to CSU/retract in the twin as the single, all while going about the normal twin training stuff. It shouldn't add any extra training hours. All that will happen is that at the end of your twin training, you will also be endorsed for CSU & retract.
It's a bit like when I started flying a Kingair 90, having never flown pressurised aircraft before. It wasn't expected that I go & get a 'pressurisation' endorsement prior to training in the aeroplane.
The biggest factor in how many hours you'll need is your current experience level (read 'skill') vs the standard you will need to be able to achieve to safely fly a twin. Not having a CSU/retract before starting twin stuff is negligible.