Prob with the Onyx 230 - has no builtin balancer so you'd need separate balancers and more wiring.

Hyperion makes a charger thats designed for charging 2 lipos at once - its basically 2 separate chargers built in one case - two sets of main charging leads and two individual balance ports. Its powerful (360watts, 180watts for each side) and expensive.

There is another charger thats been out a while thats basically just a powerful charger (350watts), A-Main sells them - Protek iCharger 208B. Some one else is building them, can't remember who offhand, but supposed to be a good charger, plenty of features and can do all chemistries. What makes it unique is the balance board - it has multiple, same size cell count balance taps and you just wire a pr of lipos together using a series connector on the main power wires and then plug in both balance connections into the balance board, set it up and charge. No additional wiring, no worries about frying a lipo if you've got balance connections wired wrong (you can short them thru the balance wiring this way if its not wired correctly).

Unfortunately this is not cheap, but its half what the Hyperion costs. The other approach as you've mentioned is to but two chargers. You can get a pr of Accucel 6 chargers and a power supply for around $135 or so shipped from HC (using their slow shipping).

I have a Flux and use a pair of Hyperion 0610iNet chargers (~$300) so I can charge two lipos at once to save time and optimize lipo performance - chargers lionk together and charge two 2S as a linked 4S and balance all 4 cells the same.