I think this would work fine:
<SyncTable Name="Users" FilterClause="[side].Username = @Username1 OR [side].Username = @Username2">
But be aware: there is quite a few things to stumble upon when designing syncscope filters. The most important thing to understand is the limitation of syncfx that it does not support "partition realignment", which is when the data changes in the filtered
columns (see here,
here and here).