Use anonymous objects to join on multiple fields:
var singOneJoin =
from prod in singOneProd.Table.AsEnumerable()
join agg in singOneAgg.Table.AsEnumerable()
on new {
Print = prod.Field<string>("print"),
Id3 = prod.Field<Int32>("id3")
} equals new {
Print = agg.Field<string>("print"),
Id3 = agg.Field<Int32>("id3")
}
select new {
print = prod.Field<string>("print")
};
Keep in mind that anonymous object property names should match.