Neither of those will work.
The first one won't work because, as you discovered, you can only use a given attribute once within a single <if> rule.
The second one won't work because mode="AND" does not check the second attribute twice, it checks to be sure that all other attributes return true. For example:
Code
<if mode="And" BuffActive="Sneak Attack|Assassin's Charge" Area="*Remnants" WeatherElement="Earth">
Would check to see if -either- SA or AC was active (because the 'OR' check is implied in how it's constructed) -and- if you were in a Salvage zone -and- if earth weather was present.
The only way to check for two buffs being active at the same time is to have nested <if> checks, such as
Code
<if BuffActive="Sneak Attack">
<if BuffActive="Assassin's Charge">
~~ both are active
</if>
<else>
~~ Only SA active
</else>
</if>
<elseif BuffActive="Assassin's Charge">
~~ AC active but not SA
</elseif>