Notion of axis (plural axes) and use of it/them is essential in dancing Argentine Tango.
For follower, she uses right or left axes. As clean weight transfer (from one axis to another) is critical to this dance, especially for follower, I usually tell beginning follower to have two good axes first.
For the lead, yes, clean weight transfer is also critical. So he should have two axes (right and left).
In addition, he can use center/spine axis that runs along his spine, with his weight distributed in between both feet/legs. Think upside-down Y-shaped structure.
When he uses the center/spine axis, it is not for mobility but for stability and support. When he uses both feet/legs to stand strong, he has much better stability, and can give her much stronger support as well as solid guide post for her moves.
As his major role in Tango dancing is to support her dance, his use of central/spine axis makes sense as an important part of this dance.
Hence, he needs to learn how to use these three axes as a leader.
What happens if she uses center/spine axis? She tends to be stable (predictably. and will be heavy), weight transfer is not clean (a major issue), and would move smaller (if she is not freeing up non-axis leg, she cannot move from where she stands). Tango becomes poorly done Kizomba of sorts.
That is why she should not (at least initially) use center/spine axis as a follow, but to focus on using two axes (right or left).
Followers are often told to "be light on your feet". It is the same thing. Either right or left, don't get stuck in between.
There are exceptions, of course, such as times the lead wants her to stand stably on both legs so that he can show off his gancho or kick through the space between her legs.
Similar moves can be done in transition, but in this case he is "placing her on the spot". You can observe and tell when he is "placing her" (or not).
You can watch Tango from this standpoint; How dancers are using their axes.
