You should be able to do it like this:
Expression<Func<BAL.Receipt, bool>> a =
x => x.InvoiceNo.StartsWith(txtSearch.Text);
Expression<Func<BAL.Receipt, bool>> b =
x => x.Alias.StartsWIth(txtSearch.Text);
List<BAL.Receipt> ac =
BAL.ApplicationInfo.db.Receipts
.Where(a)
.Where(b);