In your situation where the cart has multiple people in one request it is not only acceptable it is required... otherwise how could one manager disapprove ONLY for his person...

even if you decided to send a group approval to all of the managers of the requested for's how would you approve one item and not the others... in your case i think this is not only acceptable but required.. just be prepared in a month or two to let them know the only way to cut down the email approvals is to limit one person per cart and change the approvals... i have seen far to often when in development they say this is fine.. then when they get hit with a ton of emails the requirements change.