The sensor on the flash must be able to "see" the triggering flash, but it does not have to be direct light. The 430EX and 580EX both are sensitive enough that bounced flash works equally as well. You simply need to orient the flash sensor so that it can see 'out' from the umbrella, even if that means a convenient wall or other reflector. You will find that you can even put a flash in an adjoining room, as long as there is some ability for your triggering flash to be 'seen' by the slave unit.
For example, this image shows two 430EX flashes, used outdoors. One, being held by the biologic flash stand, has the sensor covered by the biologic flash stand attachment point (hand). The other 430EX is behind the umbrella itself, but can 'see' the triggering flash thru the umbrella and via the reflector. Note that the 430EX sensors are pointing to the side. Also, this is triggered via an ST-E2.