Haris,

Since you changed your interface, you will also need to change the class
that implements it.  If you can't change that class, you will need to change
the interface back to what it was before, and find another work around.

Assuming you CANNOT change the class and interface to be VB friendly (either
by modifying existing calls, or adding new ones), I think your only option
would be to create a small helper .dll in C# to make the call for you.  To
tell the truth, I've not created a C# dll to be compatable with VB 6... but
here's the meat of it...esentially the same suggestion I mentioned earlier,
but now using your parametrs

   [whatever attributes necessary to make COM compliant]
   class ITestingVB6Adapter
   {
       [whatever attributes necessary to make COM compliant]
       public bool TestCheck(
           ITesting testing,   // testing object to make the call against
           int param1_hi,     // high-order 32 bits of 64 bit param1
           int param1_lo,     // low-order 32 bits of 64 bit param1
           int param2_hi,     // high-order 32 bits of 64 bit param2
           int param2_lo)     // low-order 32 bits of 64 bit param2
           {
               Int64 param1;
               Int64 param2;

               param1 = param1_hi;          // assign high order bits
               param1 = param1 << 32;       // shift up
               param1 = param1 + param1_lo; // add low order bits

               param2 = param2_hi;          // assign high order bits
               param2 = param2 << 32;       // shift up
               param2 = param2 + param2_lo; // add low order bits

               // return result of object method
               return testing.TestCheck(param1, param2);
           }
   }

In VB you would call this passing your object and methods.

Dim oT As new ITesting
Dim oH As new ITestingVB6Adapter
Dim b as Boolean
Dim param1 As Long
Dim param2 As Long

param1 = 1234567
param2 = 7654321

// since VB Long's are 32 bit numbers, you can always pass 0 for high 32
bits
// if you need real 64 bit values, you will have to work out the math
somehow.
b  = oH.TestCheck(oT, 0, param1, 0, param2)

Hope this helps