Playstation 1 with weird controler port issue

Hello, I just got a Playstation 1, the original square edition, and it has this issue that hasn’t really been solved as far as I can see. I plug in a controler, the controler works perfectly fine. BUT if I plug in a second controler, or a memory unit, the controler stops working. This happends in any combination of the ports, and I have no idea what the problem could be. I have seen people online talking about the ports going out entirely because of a fuse, but this doesn’t seem to be the case here. Now, I only have one Playstation 1 controler that seems to work, sadly all, of my others seem not to, so I cannot see if it’s just that one controler. If anyone could point me in the right direction, that would be much appreciated! Thanks!