You can process 16x20 in 12x16 trays, which is handy if you don't need to do it very often. You hold the print by its shorter sides, and see-saw it through the chemical, keeping it moving continuously. (I think I learned this from AA's books, if you need a pedigree for the tip.) For a secure grip, you will need to use your hands rather than tongs, so gloves are strongly advised.
To ensure even treatment, it is probably wise to use 2 litres of chemical in each tray, though normally I use only 1 litre to process 12x16 prints in a 12x16 tray. The amounts suggested above will be music to the ears of manufacturers, but 1 litre is all you need in a 12x16 tray, provided you keep rocking the tray. Of course the less chemical you use, the more aware you need to be of its processing capacity.
Using a single-tray would be fine if you are working in a deep sink, but otherwise I'd be worried about the risk of spills from all that hurried pouring - I am fussy about my darkroom! I'd imagine your stop bath will also have a shorter life as a result of developer carry-over, but if you use an indicator stop bath that will be evident in time.