My girlfriend & ex are in business together, should I stop them?

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I have dated my present girlfriend for 3 years and I dated my ex girlfriend for about 4 years. They really disliked each other as a matter of fact my ex girlfriend would call my girlfriend to threaten her. But for some reason I kind of calm4ed the situation but for some reason they got along this year and have opened up a business in a spare room in my house, none of them stay with me though.
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I know my girlfriend can be so naïve, so I'm not worried about her but my ex girlfriend can be so mischievous sometimes. So far I am acting all cool about the business and them getting along but then again maybe I and my girlfriend are under some spell, just saying.

So far they already have clients for their small business.

I don't know if I should stop them from getting along and from continuing with the business or I should let the relationship die a natural death. I still have feelings for my ex and I know she does for me as well but I don't want her back except as a friend.

Do you think I should stop their business? Or rather stop the business in my house? Or try to break them up?