When trying to use MRT rendering to multiples color buffers:

const splatTarget = new RenderTarget({ colorBuffers: [ colorBuffer1, colorBuffer2 ], depthBuffer, })

I have a WebGl error : GL_INVALID_OPERATION: glDrawElementsInstanced: Active draw buffers with missing fragment shader output

With only one buffer, the error is gone.

It seems to come from the gsplat fragment shader not supporting MRT for regular pass.

If my understanding is correct, SuperSplat has patch this in its custom splat shader :
https://github.com/playcanvas/supersplat/blob/main/src/shaders/splat-shader.ts#L211-L212