2x GTX1050 and only two encoding streams?

Hello, my configuration

HW: 2x GTX1050

SW: Linux stream 4.4.0 #1 SMP Fri Oct 14 22:04:02 CEST 2016 x86_64 x86_64 x86_64 GNU/Linux
cuda_8.0.61.2
nvidia-384
NVAPI (R384)
ffmpeg version N-87023-g8754ccd

I use for video encoding with hevc_nvenc codec.
I’m able to streaming two streams at one time. I can specify GPU via -gpu N ffmpeg argument. It works.
I can switch both encoding processes between GPU 0/1.
But I cannot make more than two streams.

Is it limit of the NVAPI, nvidia driver, ffmpeg or any fixable bug?
When I try run 3rd instance of encoder, I get out of memory (10) error.

[hevc_nvenc @ 0x3ec6ba0] Loaded Nvenc version 8.0
[hevc_nvenc @ 0x3ec6ba0] Nvenc initialized successfully
[hevc_nvenc @ 0x3ec6ba0] 2 CUDA capable devices found
[hevc_nvenc @ 0x3ec6ba0] [ GPU #0 - < GeForce GTX 1050 > has Compute SM 6.1 ]
[hevc_nvenc @ 0x3ec6ba0] OpenEncodeSessionEx failed: out of memory (10)
[hevc_nvenc @ 0x3ec6ba0] [ GPU #1 - < GeForce GTX 1050 > has Compute SM 6.1 ]
[hevc_nvenc @ 0x3ec6ba0] No NVENC capable devices found
[hevc_nvenc @ 0x3ec6ba0] Nvenc unloaded

Thank you for answer.

The current SDK package allows up to two simultaneous encode sessions per system for low-end Quadro and GeForce cards. (check https://developer.nvidia.com/video-encode-decode-gpu-support-matrix).