If the federal employee who generated the purchase request has any type of relationship with the other federal employee, there is a potential for a conflict of interest. If the federal employee who provides the item or service during their non-duty time has no personal or professional connection to the requester— and there is a good business case to make award to a supplier who happens to also be a federal employee— there might not necessarily be a conflict of interest. However, I am not legal counsel. In order to proceed with confidence, you need to check with your organization's legal counsel.
Update: for additional guidance and requirements for contracting with Federal Employees, please see FAR subpart 3.6 Contracts with Government Employees or Organizations Owned or Controlled by Them. You will also want to check your organization's (USDA) FAR Supplement to see if you have additional implenting or supplementing langauge on this topic.