This sounds like a limitation of your hardware, not the game. Some keyboards have limited N-Key-rollover capabilities.
https://gaming.stackexchange.com/questions/6669/how-do-i-remove-the-limit-on-pc-...for more info
Wow, never knew this, will double check this. I never had any issues with games before though, but I can also not with 100% certainty say that I have played games with 'twin stick setup' lately. Will double check this and inform the dev here.
