There have been developments in this area in recent years Luke. Nobel Biocare have released an All-on-4 product and other manufacturers have released similiar products. There have been some encouraging clinical tests but as the systems are approximately 5 years old, there is a lack of long term data available. Early indications are positive though!!
we've just learned procedures where we implant to implants in the mandible (or maxilla) to attach a denture that is removable, but is tightly attached for funciton and everyday use. They look soooo natural and don't 'slip' like regular dentures do. We've learned the 2 implant technique although some practictioners opt to place more than 2....
I know I have severe oral pathology, but it has been stable and even regressive over the past 10 years. ( I am 22) So I am hopping as long as I have good bone mass, that I can get some fixed appliances as I am too yong to deal with dentures or removable bridgework, as they both lead to further destruction of bone.
Luke, yes it is possible to have upper and lower implants done with 3-4 unit bridges separately instead of a whole arch with crowns.. (in case of implant failure which is a low percentage). you would need panoramic x ray, study models etc. to study your case before starting.