Hi Calusa,
In answer to your question the answer is yes it is possible, while i personally wouldnt reccomend it, As william said the bigger the water volume the better marine fish need good quality and stable water conditions any changes to water quality would rapidly effect such a small voulme of water,
However this is how i would set up a tank that small, there is no point using a dsb (deep sand bed) as this would take a lot of space, i wouldnt use an under gravel fiter as these arent as effective as once thought i would use an external filter, which i would clean regularily, as they can become nitrate factorys if left to long, i would have a few pieces of live rock in there to aid filtration,
Finally the two clowns would be fine in there, i wouldnt add any other fish i would suggest maybe a cleaner shrimp and a few turbo snails for added interest and the also aid keeping the tank clean.
While water testing every day isnt necesarry i would perform water changes every few days, also i have seen plans for a skimmer suitable for a small tank if i find the link i will post it here.
Of course there is always the possibility of adding a sump to the tank which could increase the volume of water in the system
Hope thats of help