But you can legally declare a charge on 2 BGs in the first place provided that they are both in charge reach.
However, you must be able to contact both (with a step forwards if necessary) in the position they are currently in.
i.e. if neither evaded. You can't declare a charge on a Bg that you could only contact if another BG evaded.
from the rulebook:
To be allowed to declare a charge, there must be a visible enemy base that can be “legally” contacted by the charging battle group within its normal move distance through the terrain to be crossed. A battle group can declare charges on as many enemy battle groups as can be“legally” contacted within this move distance.
and:
Any enemy battle group in the path of a charge counts as being charged if it can be “legally” contacted, even if it was not one of the originally declared targets of the charge. This applies even if it can only be contacted by bases stepping forward. It does not apply if, due to intervening friends, it could not be contacted even by stepping forward bases – unless the situation changes ..........
Since the situation can't change until after the evade - to declare a charge on multiple BGs you MUST be able to contact each target before the evade.
To address another point:
I think this is not quite true. I believe the Cav can declare a charge with a slight wheel so that they will hit only the LH if the LH don't evade, but will hit the MF if the LH do evade. In this case a charge is only declared against the LH untill they evade at which point a charge is declared against the MF if the cav's actual move will reach them.
The subtle difference is that if different charge is resolved first the MF are eligible to intercept it.
You have to be careful about timing with regard to charges, evades and intercepts....
The sequence is:
Declare all charges.
Make interception moves.
Make evade moves.
Make charge moves.
(missing out the bits requiring CMT's/break tests etc)
So you must:
1) Declare charges - (on targets that are legal before any bases are moved)
2) Make your interception moves - (note that this is BEFORE evades)
3) Make evade moves
4) Move charges
A BG that becomes a target because of the evade (and only if the VMD roll takes the chargers into them), only tests for being charged between points 3) and 4)
It's also important that intercepts are made before evades - which means that you can't intercept a charge if it only becomes possible after the evade.