The problem are the RTL2832U receivers. The sample rate is too low, 500ns resolution with 2MSample/s is not enough. For this reason I cannot recommend using the dvb-t sticks as ads-b receiver (the symbol rate is too high, compare nynquist theorem).

Multilateration has already be done for ADS-B on large scale, but with different receivers. There are receivers available[1] with nanosecond accuracy (and gps time synchronization).

GPS synchronization isn't needed, by choosing other flight paths to provide basic time synchronization between receivers.

[1]: <http://radarcape.com/>