Hello,
I would consider a single 10Gb link to be adequate for FT logging traffic in most cases, but the throughput depends on how busy a VM (and how many), a second link for redundancy might be prudent, over another switch if possible.
Not often to recommend a TT article but this one is good and has a formula for estimating the networking requirements and summarizes the FT section of the vSphere Availability guide quite well:
http://searchitchannel.techtarget.com/feature/VSphere-Fault-Tolerance-requirements-and-FT-logging
VMware FT logging bandwidth = (Avg disk reads (MB/s) ×
8 + Avg network input (Mbps)) × 1.2 [20% headroom ]
The vSphere Availability guide has more detailed info that informs us that yes, you should put FT logging on it's own Multi-gigabit link:
"Prerequisites
Multiple gigabit Network Interface Cards (NICs) are required. For each host supporting Fault Tolerance, a
minimum of two physical NICs is recommended. For example, you need one dedicated to Fault Tolerance
logging and one dedicated to vMotion. Use three or more NICs to ensure availability.
NOTE The vMotion and FT logging NICs must be on different subnets.":
Bob
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