[IfritnoItazura]

The reason for WHM to Lv.18 first is because WHM with no support job isn't all that much less than WHM with support job, while BLM/WHM provides a lot more utility than BLM/---. (Cure is that important at low levels...)

In overall scheme of things, either leveling BLM or WHM first is OK, I suppose.

* * *

Switching between low level BLM and WHM isn't that bad; the main differences are the INT/MND rings, neck piece, and... er... Hands?

You can level them concurrently, and decide what to level each night base on search results. For example, suppose you are Lv.12 for both WHM and BLM:

/sea all 11-13 inv BLM
(Returns a list of all BLM's between Lv.11 to 13 looking for party.)

/sea all 11-14 inv WHM
(Returns a list of all BLM's between Lv.11 to 13 looking for party.)

If plenty of WHM's seeking, you may be better off seeking on BLM instead, and vice versa.