The issues would primarily depend on separation distances for the septic tanks from the individual houses and if you would have space for the required percolation area. There may also be planning issues which could cause a problem with getting planning for another house (housing need, ribbon development, minimum site size, acces onto road etc). you should check the local development plan, and look for a pre planning meeting with you local planning officer to see if it is feasible, before you spend any money on architect, and the site suitability testing