Conflict between two-handed weapon and offhand equipment is ignored when you are in spell-casting mode instead of melee attack mode. Therefore two-handed weapons can be equiped together with either a shield or a single-hand weapon (at least until it is patched).
Equip a two-handed weapon together with a shield:
1. Equip a shield, leave your main hand empty
2. Select a spell instead of melee attack as your action
3. Equip a two-handed weapon onto your main hand
Equip a two-handed weapon together with a single-hand weapon:
1. Equip two single-hand weapon onto both hands
2. Select a spell instead of melee attack as your action
3. Remove the single-hand weapon from your main hand, then equip a two-handed weapon onto the main hand
If you wield a two-handed weapon together with a single-hand weapon, you will still be able to initiate Dual-Wield powers such as Wave of Force and Elemental Rage, and at the same time you still benefit from the two-handed weapon skills, such as the chance to stun bonus