Yes, you can do this, but of course the pressure in the 48l tank gets lower each time.
The total amount of free gas stays constant, so the relevant equation for starting with an empty 12 ltr tank is
Start Pressure * 48 ltr = End Pressure * (48 + 12) ltr
So the pressure goes to 80% of starting pressure when filling an empty 12 liter tank. For example, if the 48 ltr starts off at 300bar, then both the 12 and 48 ltr tanks will be about 240 bar after fill the 12. (actually a couple percent lower due to nonlinearity in volume vs. pressure -- air up at 300 bar doesn't quite follow the ideal gas curve). The 2nd fill of an empty 12 liter will drop the pressures to about 190 bar, and a 3rd fill gets you only 150 bar.
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If you start with a partially filled 12 liter tank, the equation is a bit messier, but still straight forward.
Starting free gas = ending free gas
Start bars in 48 * 48 ltrs + start bar in 12 * 12 ltrs = end pressure * (48+12).
If you are serious about doing this type of filling, look into a cascade system. This is where you use multiple 48 liter bottles (typically 2 to 4). Use the lowest pressure 48 liter to start off the fill, use the highest pressure 48 liter to just finish the final top off. That way the highest pressure 48 liter bottle stays up at higher pressure for more fills.
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After you work out the numbers you'll probably just go buy a few more 12 liter tanks to swap out between dives.
Charlie Allen
