combining two individually-defined dapper queries

DapperCombination.cs

interface IDapperQuery<out TResult>
{
    string GetSql();
    DynamicParameters GetParameters(); // <--- can't make this work
    IDictionary<string, object> GetParametersAsDict(); // this works
    TResult MapResult(SqlMapper.GridReader reader);
}

// Combines both queries for one DB round trip
class CombinedQuery<T1, T2> : IDapperQuery<Tuple<T1, T2>>
{
    private readonly IDapperQuery<T1> query1;
    private readonly IDapperQuery<T2> query2;
    public string Prefix1 { get; set; } = "q1_";
    public string Prefix2 { get; set; } = "q2_";

    public CombinedQuery(IDapperQuery<T1> q1, IDapperQuery<T2> q2)
    {
        this.query1 = q1;
        this.query2 = q2;
    }

    public string GetSql()
    {
        // Combine two sql statements into one.
        // Parameters get prefixed, e.g. @myParam becomes @prefix_myParam
        string sql1 = query1.GetSql();
        sql1 = ReplaceParameters(sql1, query1.GetParametersAsDict().Keys, Prefix1);

        string sql2 = query2.GetSql();
        sql2 = ReplaceParameters(sql2, query2.GetParametersAsDict().Keys, Prefix2);

        return string.Format("{0};\n\n{1}", sql1, sql2);
    }

    private static string ReplaceParameters(string sql, IEnumerable<string> paramNames, string prefix)
    {
        foreach (var name in paramNames)
        {
            string original = "@" + name;
            string replacement = "@" + prefix + name;
            sql = sql.Replace(original, replacement);
        }
        return sql;
    }

    public DynamicParameters GetParameters()
    {
        // Cannot do this with current public interface
        throw new NotImplementedException();
    }

    public IDictionary<string, object> GetParametersAsDict()
    {
        // combine the 2 dictionaries into 1 with the prefixes added
        var combined = new Dictionary<string, object>();

        foreach (var kvp in query1.GetParametersAsDict())
        {
            combined[Prefix1 + kvp.Key] = kvp.Value;
        }

        foreach (var kvp in query2.GetParametersAsDict())
        {
            combined[Prefix2 + kvp.Key] = kvp.Value;
        }

        return combined;
    }

    public Tuple<T1, T2> MapResult(SqlMapper.GridReader reader)
    {
        var first = query1.MapResult(reader);
        var second = query2.MapResult(reader);
        return Tuple.Create(first, second);
    }
}