The load will be nominally half the power, but can step up to full power and then back to nominal often.

I plan to use an active PFC with an isolated DC/DC converter output. Instead of making one large power stage - I'm leaning towards building two converters and paralleling the output stages. Or making one AC/DC (PFC) stage and two DC/DC converters with paralleled output. I'm interested in the Intersil PFC chip (ISL6730). They have an app note for a 300 Watt PFC stage that I could implement and each of these run a 300 W DC/DC stage which would be connected in parallel.

I am also looking at Fairchild and TI PFC devices. I haven't decided on how to do the DC/DC stage. To parallel the outputs, TI and ST have parts for this.

Any comments or suggestions?

I'm curious as to why you would use two DC - DC converters instead of one. Is it because the lower power converters are already designed? It would seem to me that a single 540 watt converter would be less trouble and cost than two converters with the issue of forcing load sharing.

I suspect that the physical size of the semiconductors will be about the same for a 300 watt converter as for a 540 watt converter. The magnetic components will be larger but probably less than two sets for the lower power converter. Also you only have to deal with one PWM controller and no load sharing circuitry. As for topology, the clamped forward converter has a significant advantage if the input voltage has a large variation. Since you are going to use an active PFC front end, the input to the DC - DC section will essentially be regulated so the clamped forward converter really doesn't have an advantage over other topologies.

To add the Marshall´s comment, interleaving also reduces input ripple - not as important in a boost, but interleaved DCM or boundary mode (the variable frequency, sit-at-the-border of DCM-CCM method) boosts reduce peak currents, meaning lower RMS, so everything´s cooler and more efficient.

Linear Tech has had interleave controllers for some time and I believe they are the innovators. They referred to it as polyphase. Lots of buck controllers but not sure about PFC. Worth looking into.

I just finished a four phase 1400 watt PFC using a microchip digital controller. It was CCM and its efficiency was well into the mid to high 90's. But that's probably overkill, TI does have a dual phase boundary mode controller that would work well.