If two people form an LLC for a fitness training business (were previously fitness trainers and each had their own clients), can they keep the old clients separate and not include those in LLC income? The LLC is taxed as an S-Corp. Would they need to report the income/expenses related to the old clients on their personal returns on Schedule C, separate from Schedule E for the LLC income/loss?

Basically, can they have an LLC while also continue training old clients on the side that would not be part of the LLC?