Some hens are fine with co parenting others not so fine. Cochins and silkies are generally mellow and the fact that these two are 'buddies' should help. The problem with hens setting on eggs in the same nest is that they tend to 'steal' eggs from one another and inadvertently cracking or breaking them.
What about store bought chicks and slipping them in with the broodies?
My best success with that has been hens that have been setting for close to 21 days. Place the chicks under the hen in the evening so that the hens can bond to them over the course of the night.