It's better to build two pre-equalized anchors on top of each other when working in series. However, occasionally this isn't possible and you're forced to work with a magic x. When that happens, make sure that the pieces that are not a part of the x are extremely strong.
An Anchor In Series with a Pre-Equalized Knot on the Right-Hand Leg
This anchor is essentially a three piece anchor that was linked together in series
because the climber only had two double-shoulder length runners to build an anchor.
It is quite possible to build a vast anchor with codellettes and slings in series. And sometimes -- when the rock is very bad -- that is exactly what you have to do.
There are many other ways to add additional pieces to an anchor and to keep it SRENE, but for many who are just dipping their toes into the world of leading, anchors in series make a lot of sense. Most guides recommend that beginning level leaders work with anchors in series for a significant period of time before experimenting with other systems. This will help lay a solid intellectual framework of what an anchor is supposed to look like and what it is supposed to do.
--Jason D. Martin