on Bar.Id equals Foo.BarId1 or Bar.Id equals Foo.BarId2

var data = (from f in Foos
                join b in Bars on f.BarId1 equals b.Id into tb
                  from xb in tb.DefaultIfEmpty()
                join b in Bars on f.BarId2 equals b.Id into tb2
                  from xb2 in tb2.DefaultIfEmpty()
                select new { Foo = f, Bar1 = xb, Bar2 = xb2 });

var data = from f in Foos
            from b in Bars.Where(x => x.Id == p.BarId1 || x.Id == p.BarId2).DefaultIfEmpty()
            select new { Foo = p, Bars = b };

Foo Bar
f1  b1
    b2

Foo Bar
f1  b1
f1  b2

var data = from f in Foos
       select new
       {
           Foo = f,
           Bar1 = Bars.FirstOrDefault(x => x.Id == f.Bar1Id),
           Bar2 = Bars.FirstOrDefault(x => x.Id == f.Bar2Id)
       };

var data = (from f in Foos
                join b in Bars on f.BarId1 equals b.Id into tb
                  from xb in tb.DefaultIfEmpty()
                join b in Bars on f.BarId2 equals b.Id into tb2
                  from xb2 in tb2.DefaultIfEmpty()
                select new { Foo = f, Bar1 = xb, Bar2 = xb2 });

from f in Foos
select new { f, f.Bar1, f.Bar2 }

var data = from f in Foos
           from b in Bars
           where f.Id == b.BarId1 || f.Id == b.BarId2
           select new { Foo = p, Bars = bx };