Simple Q&A D&D 3.5 (by RAW) XXVII

[Gildedragon]

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The buckler-axe makes no exception for itself because it never stated that it had to use the buckler rules.

Fair enough. Then you cannot hold a weapon in the same hand as the buckler axe in that case.

If it behaves like a buckler, you take the penalties to attack and lose the ac bonus if you attack with a weapon held that hand.
If it does not behave like a buckler, you do not get to hold a weapon in that hand.