robbing from the positive rail

[Merlinb]

is using only 2 diodes per rectification location as good as 4 (or a bridge rectifier)?

You always have two diodes per location: two for the positive, and two for the negative. Plus another two for another positive rail. You can use single diodes or bridge rectifier packages if you want (in which case one of the the packages would contain two unused diodes); it's up to you.
However, you have not said what it is you are trying to power, so you may be getting quite inappropriate suggestions from people...

 

[tinitus]

This one is better...

obvious and logic

would it also work if using double bridge ?

 

[richie00boy]

Post 13 and 19 are actually identical in practice, just look what get's shorted out... With a centre tapped transformer it's not possible (well it is, it's just a waste) to use twin bridges.