When replacing a whole chicken with two chicken in a slow cooker recipe...?

Should I cut down the amount of time to cook it? For a whole chicken, the recipe says high for 4 to 5 hours. I'm just making it with two boneless chicken s though. I don't want to burn the meal, but I don't want to under cook it either... If you need to know, it is spanish chicken so the wet stuff includes a tomato paste mixed with beer. That's pretty much it (besides sprinkling salt, pepper, paprika, and garlic powder on the chicken ).