I have 1 - 5 year old and 1 twins - 15 mths. They are taken care by my 2 helpers everyday. One of helper is smart and good at time management. The other one is not smart but is willing to take instructions. They both love my kids.
They have taken care the 3 kids since the twins were born. So far, it is ok but I am not a picky one. The not smart one does not like to take my elder one to the interest class though it is a comparatively easier task. She prefers stay in the house everyday. It is fine when the babies were small.
Now, they grow up a bit and become very very active. I am going to change the not smart one because they really need someone who has better time management skill and can bring them to playgroup.
All in all, I am a lucky one and my helpers are good and can serve my purposes so far.
Yes, a bit of too risky. But it is quite impossible for 1 to take care twins all the time. If the twins are crying/sick at the same time, it need the others to help.
You better talk to your first helper and ask her to offer helps when necessary. I did pay more to my smarter one. You may consider that too. Secondly, you may ask your helper to refer someone that she can work with and is willing to take care twins. By doing that, they will cooperate with each other.