Yes, I have done this on all my Keihin equiped bikes. Split them up and run the two that come from the vertical fittings either in the top of the airbox or into the frame backbone. The other three remain routed low. To go one better get a check valve/filter kit and install it as well. With the stock vent hoses all low, the carb will suck water and muck during long water crossings.
Although I haven't yet seen much deep water here in Mississippi, I did the same as GlennP, just a habit with me. Plus, as I understand it from the magazines this will also help prevent bogging in whoop sections as it prevents the fuel from frothing up.
I actually have now gone a step further and installed filtered, one way vents and a vent on the crancase breather as well (routed up to the backbaone). I purchased mine from http://www.kevinscycleracing.com/advert.html#page4 Look for the Deep Water section at the top of the last page on the web site. They are really into enduro type stuff; very good people to deal with.
'97 KTM 250 EXC, '85 XL-600
AMA, Blue Ribbon Coalition, NETRA, SERA (soon)
After looking at Mike Lafferty's enduro bike he had parked behind his race van at the GNCC in Macon Georgia, This was the bike he had just wone the Alligator a few days before. The two top lines out of the carb were routed up in to the breather. The other lines were left down with no check valves. just open ended.