You need tro upgrade your energy network to handle the increased throughput.
Since you are feeding GregTech machinery, I am assuming you have a MV transformer turning the 512 EU/p from the MFSUs into 4x 128 EU/p, to feed blast furnaces and electrolyzers. And depending on which machines, you might further have a LV-transformer going 128 EU/p -> 4x 32 EU/p.
However, transformers are throughput chokes. They can only perform one transformation per tick. Either they can accept four small packets and make one large, or they can accept one large and send four small. The result is that no MV-transformer will let through more than 512 EU/t per tick, no matter how many sources you have connected and what packet sizes they are sending or what direction you are transforming. The same goes for the LV-transformer, which is limited to 128 EU/t, and the HV-transformer, which is limited to 2048 EU/t.
Your second MFSU is not outputting because the first one is fully satisfying the needs of your single MV-transformer. Build a second one in parallel to the first, and you'll see both MFSU's output.