just paralleling up the power supplies will probably not result in any
power exceeding the power supplied by the transformer with the largest
swing.
let me describe this for you:

your rectifiers act as switches, whenever the voltage across them
exceeds 0.6 volts (for silicon diodes), they will conduct. now you
connect the anodes of two rectifiers that are driven from the other
side by a voltage generators. If you imagine that one of the two
diodes are driven by a voltage source that is higher than the other by
0.6 volts, then the diode will never switch on as the other diode will
reverse bias this diode. so, simply paralleling up the dc outputs may
not work. A 0.6 volts difference between two transformers is almost a
given.

my recommendation to you would be to wire up the primary windings in
parallel and the secondary (18v) windings in series. That way, if the
phase is proper you will get 18-0-18v (36 v) across the two
secondaries. A bridge will get you 18V.

Something like this :

----+-3|
| 3|C--------------||---+
| 3|C |
| 3|C------+ |
-+--|-3| | |
| | | +-------+----O 18 DC +v
| | |---- Gnd | |
-|--+-3| | | ===
| 3|C------+ | === filter cap.
| 3|C | |
| 3|C--------------||---+ Gnd
-+----3|

this uses just two of the four transformers. besure that the voltages
of the secondary are in phase. (you will not get more than a volt or
so if the are not). there is probably a way to also utilise four
transformers, it is in the corner of my eye, but the above given is a
cookbook recipe.
However, if you require 36v, then it is a simple matter to extend this
by using two supplies with secondaries in series on each side of the
rectifier bridge.

- farhan