First you need a set based on your normal WS set + your relic feet:
Code
<set name = "2Hour" BaseSet = "WS">
<feet>Saotome Sune-Ate +2</feet>
</set>
Then you need a rule that looks for yor two hour being up:
Code
<if Type = "WeaponSkill" BuffActive = "Meikyo Shisui">
<equip when = "precast" set = "2Hour" />
</if>
There are multiple ways this can be done, though. If you have different WS sets for different WS it might be simpler to add this at the end of each WS rule (after the other WS gear has been set):
Code
<if BuffActive = "Meikyo Shisui">
<equip when = "precast">
<feet>Saotome Sune-Ate +2</feet>
</equip>
</if>