Absolutely, unquestionnably a two light system. If only for redundancy if one craps out. Spot on the head, flood on the bar. Spot to see where you're going, flood to provide contrast. Make sure your flood does not overpower your spot.
I have a self contained light, and I think they are too heavy and would be susceptible to bouncing around a lot. But I haven' tried it off-road yet, so i can't say definitively.
If you don't want/need long run times, you can get smaller battery packs (2-cell vs 4-cell vs 6-cell). When I ran a one LED system on my helmet, a 2-cell battery would get me through a 2 hour ride on high most of the time. And I could mount it on the helmet for a nicely balanced feel.
I find 4-cell systems are perfectly fine weight wise though. I used to lug around a sealed lead-acid house alarm battery back in the day, so anything is an improvement I guess, but I don't find the weight of the new batteries to be a problem.
Honestly, take a close look at the ITUO systems. They're well priced and well made. you could get an XP2 with spot spot on the helmet, and an xp2 with flood flood for your bars. Get a 2-cell battery for the helmet and a 4-cell for the bar (that way you have a bit of backup run-time if you need it).
Thanks. What's the pricing info for those ituos? Couldn't find anything on the website