Capacitors in series
Capacitors in series are like resistors in parallel, to get the total capacitance add the reciprocals, then take the reciprocal of the result.
For example, if we have 20pF, 10pF, and 5pF caps, the total capacitance would be:

1 / ((1 / 20pF) + (1 / 10pF) + (1 / 5pF)) = 2.857pF

For two capacitors you can use the (C1 * C2) / (C1 + C2) but this won't work for 3 or more.  

                 20pF 10pF 5pF
Connect here -&gt; -||---||---||- &lt;- and here

                    2.857pF
Equivalent to:  ------||------


Note that putting multiple caps in series probably won;t work very well at such a low value (there will be extra unwanted inductance which will lower the resonant frequency)
2.2pF is actually a very common value, for example Farnell has 46 options under RF caps and 94 options under ceramic caps. I advise picking one of these.