By datastore, you mean NFS/iSCSI?
Is possible to team on ESXi, no problem at all. I really son't think you will have enough network traffic in only one ESX to use more than 1GBit, but feel free to do the teaming - in my point of view a teaming will be useful for failover, but will not increase the speed of accessing the datastore.
Generally, nic teaming works better if your switch supports LACP or etherchannel or some link aggregation methods. Without this, hardly you will get more than 1 nic really used.
As an example, a single tcp connection between 2 computers never will use more than 1 nic, neither with etherchannel or LACP - if you need more speed a 10GBit nic would do a better job.
In your case, I would recommend doing the cross over cable - or use a switch for failover purposes.
Marcelo Soares
VMWare Certified Professional 310/410
Virtualization Tech Master
Globant Argentina
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