If it's an FSA, you may be able to contribute up to $2500 to each company's plan.
See the IRS reference in this http://www.bogleheads.org/forum/viewtopic.php?f=10&t=101267&start=50#p2147152.
kcb203 wrote:5) Putting anything else into taxable.
New annual Contributions
$17,500 his 401k (plus 4% employer matching)
$5,000 his Roth IRA
hicabob wrote:Vanguard requires you go through the Medallion Signature process to do the same, but other than that it's straightforward enough.