Assuming you connect the two 12V secondary windings in parallel, then the rated transformer output will be 12V AC RMS @ 8.3A RMS.
If you connect in series, you will have 24V AC RMS @ 4.1A RMS.
However, the relationship between the AC & DC voltages and currents will depend on how you setup the rectification and smoothing, and the nature of the load.
This leads to the "Transformer Utilization Factor" which is defined as the ratio of DC power, to AC VA load on the transformer. You'd hope this would be 1, but in most simple rectifier circuit it is much lower.
The absolute maximum values (assuming perfect diodes) for the common setups are as follows
- Half-wave rectifier ~0.3
- Centre-tapped rectifier ~0.7
- Bridge rectifier ~0.8
(Detail of how these values are calculated can be found at the above link.)
Real world losses (such as diode drop) will lower this further.
In your case, a 100VA 12V transformer with a bridge rectifier should therefore not be expected to deliver more than 80W @ 12V DC, which is ~6.6A, and allowing for diode losses will probably be nearer 75W.