We have a C#/.Net 4.0 application which imports RSA Private Keys from a String in Base64 received in a WebService.
This application works perfectly for RSA-Keys in 1024 bits, but doesn't with a special kind of rsa private keys (around 1% of keys).
Here are the byte lengths:
RSACryptoServiceProvider has some assumptions on the data lengths which are:
<= 4bytes; though RSACng allows "any size"), let's call the length
So, assuming that your second key is actually Modulus: 128 bytes (because a 64-byte P times a 64-byte Q isn't a 256 byte number), you just need to left-pad the D array with a zero to bring it up to the proper length.
byte newD = new byte[modulus.Length]; Buffer.BlockCopy(d, 0, newD, newD.Length - d.Length, d.Length);
.NET Core has the source code available showing that relationship. In .NET Framework it's buried inside the CLR, so not available on referencesource.