So far I've used an Whirlwind IMP2 and a Behringer. They've both worked fine, but I liked the IMP2 better because it's smaller. Whirlwind makes a Jensen passive DI box that I might try. I'm also considering having a Jensen transformer installed in my DS5R, with in and out jacks on the rear and an XLR on the front, just so everything is self-contained.
The SF2 can be run mono or stereo. I'm running it mono, so it's more of a system eq. The 2402 has a parallel input mode, which means it can take one signal from the channel 1 input and send it to both channels of the amp. One side runs 1 or 2 Eden 12's, and the other runs 1 or 2 Bergantino 12's, depending on the size rig I need. That's one of the reasons I haven't tried one of the new 1804's...they don't have the parallel input feature (or the clip limiting, or the hipass filters). I could just make a cable, though.
Hosa (among others) makes a Y-cable that would work, too, if you'd rather just buy one. They're less than 10 bucks. You need a 1/4 mono male to 2 1/4 mono males. That way you can use both cabinets all the time instead of dedicating one to each pickup. Should give you substantially more headroom if you ever solo a pickup. If you run them both pretty close to full all the time, it won't make nearly as much difference.
Try this:
Neck to channel 1 of F2B
Bridge to channel 2 of F2B
Mono out to DI
Single end of Y cable out of DI
Each of the other ends into it's own channel of the amp.
If you're not using a DI, you can just use the Y cable to go from the mono F2B out into each channel of the 1804. Also, each side of the amp will still get it's own speaker, but since each side is also getting the same signal, they'll have the same output regardless of your pickup volume settings.
Hope this helps,
Charles